The last person to interview Gloria Steinem for the Observer was Martin Amis, in 1984. He waited for her at the offices of Ms, the magazine that she co-founded in 1972 – "Pleasant though I found it, I was also aware of my otherness, my testosterone, among all this female calm" – and then they headed out together to Suffolk County Community College, Long Island, where Gloria was, as ever, to address a group of students. To read this piece now is excruciatingly embarrassing, especially given Amis's more recent conversion to what he likes to call the "gynocracy". Feminism? From the male point of view, he said back then, the reparations look to be alarmingly steep. As for Steinem herself, she is "the least frightening" kind of feminist, being possessed of – prepare to be amazed! – both a sense of humour and good looks. She was, he wrote, relief slowly blooming, "nice, and friendly, and feminine... the long hair is expertly layered, the long fingers expertly manicured. Fifty this year, Ms Steinem is unashamedly glamorous."

A quarter of a century later, and Steinem is still glamorous: wildly so. But the point is surely that this glamour derives, just as it always did, as much from her extraordinary career – in other words, from her brain – as from her appearance (Mart unaccountably failed to spot this). At 77, she remains tiny of waist and big of hair – and, yes, the nails are as smooth and as shiny as a credit card – but what strikes you most, at least at first, is how preoccupied she seems. She is so busy. It has taken me the best part of two years to bag this slot with her, and even now I'm here, I'm uncertain how much time, in the end, she will have to spare. Does she remember who I work for? I can't tell. I have the impression that she believes I live in New York – and sure enough, when I eventually tell her that I've flown in from London, she looks first amazed, and then, quickly, solicitous. (She might be distracted, but Steinem is also famously nice.)

We are in her flat on the Upper East Side, a womb-like, slightly hippyish basement stuffed with velvet cushions and piled polemics (my going away present, plucked from one of these piles, will be a slim volume by her former Ms colleague, Robin Morgan, called Fighting Words: A Toolkit for Combating the Religious Right). It's a swanky address but, as she points out, she bought it aeons ago, when even such lowly forms of life as political activists and freelance journalists could still afford a piece of Manhattan real estate. In one corner of the sitting room is her desk, lit by a single lamp; in the other, the desk of her assistant, Amy, who also sits beneath a neat halo of light. Here, two sunbeams in the troglodytic gloom, they drink Starbucks, fire off emails, write books, and generally plan the next stage of the revolution. Visitors like me, though welcomed with pomegranate juice and, when this is discovered to have run out, coconut water, are an unwelcome distraction from the main event, which is work. "I hope to live to 100," she says. "There is so much to do."

Her looks, though. Let's get them out of the way first. She smiles. Before she joined the women's movement, she was merely "a pretty girl" (not that she necessarily thought so: her famous aviator shades were, she says now, something to hide behind, and her streaked hair a tribute to Audrey Hepburn's turn as Holly Golightly, Truman Capote's country bumpkin-turned-cafe society girl – a character to whom she "totally" related). Afterwards, she was "suddenly beautiful", and though the attention this brought was occasionally useful, mostly it was just a pain in the butt: the tiresome suggestions that she had only got on thanks to her appearance; the hurtful ire of that other great feminist, Betty Friedan, whose loathing of Steinem seemed mostly to be motivated by envy. In the early days of Ms, a local pornographer stuck a huge picture of a naked woman on his building. This woman had Steinem's face and Steinem's hair and, from a few blocks away, looked as though she was surrounded by floating pink salamis. Up close, however, the passerby would soon realise his mistake. "Pin the cock on the feminist," said the poster. Steinem had to look at it every morning as she arrived at work.

And now? When she looks in the mirror, what does she see? "Well, I'm shocked. Sometimes, you're passing a store, and you see this person in the window, and you think: who is that? Oh, it's me. But I've also realised that ageing is a bit like what being pregnant must be like, by which I mean that your body knows how to do something that you don't know how to do – and it's quite interesting. Your body loses what it needs to support someone else, and it keeps what it needs to support you. That's very smart. Just watching the process is somehow fascinating."

A few years ago, during a brief stint hosting the Today show on NBC, she had a little fat removed from around her eyes so, as she once put it, "I didn't look like Mao Tse-tung and I could wear my contacts." But she looked worse afterwards. "And what I care about is the message, and I realise that if I had plastic surgery, it would just distract people. It would be like having a bad toupee; they wouldn't listen." She is appalled by America's obsession with cosmetic surgery – "I keep thinking: Georgia O'Keeffe wouldn't have had Botox" – and would like to mount some kind of campaign against it: a warning, perhaps, of its side effects. "It is part of an obsession with youth, but it is also pushed – and this is much more reprehensible to me – by pornography, in which women are made to look like children. What has sent me over the edge is this operation in which the labia minora is tucked in. That's what they think women should look like. It's horrifying. It normalises abnormality." Straight-backed in her armchair, she grimaces theatrically.

Steinem is presently hard at work on a long overdue book of inspirational essays about being on the road – rarely a week goes by without her taking a plane – but in the meantime, she has been popping up elsewhere: an HBO documentary, Gloria: In Her Own Words; a long "oral history" of Ms, in honour of its 40th birthday, in New York magazine (among other things, this piece revealed that possible names for the ground-breaking Ms included: Sojourner, Lilith and, er, Bimbo). The film – it has not yet been screened in the UK – is riveting. Those who think of Steinem and see only a placard and a megaphone will be startled to see her younger self ironing, discussing her marriage prospects, and tap dancing in an elevator.

She is an extremely good tap dancer, having once believed that hoofing was her ticket out of Toledo, Ohio, the city where she grew up. Her childhood was difficult. The family was poor, and lived and travelled for a time in the trailer her father, Leo, used to ply his trade as an antique dealer. When Gloria was only a few years old, her mother, Ruth, had a nervous breakdown, and it left her an invalid, "someone who was afraid to be alone, who could not hang on to reality long enough to hold a job, and who could rarely concentrate long enough to read a book". In the fullness of time, her much older sister having left home, and her parents having separated, Gloria became her mother's full-time carer. She was then just 10 years old. When she finally escaped – having graduated high school, she won a place at Smith College, Massachusetts – she was so determined not to be "sucked back in", she resisted travelling to see her father even on his deathbed, fearful that if he recovered, she might end up having to care for him, too. He died alone, and she still regrets this.

In 1960, having moved to New York, she began working as a journalist. It was tough. If finding an apartment was difficult – a single woman who could afford to pay her own rent was clearly a hooker in the eyes of most landlords – winning serious work proved to be well nigh impossible. Editors thought female employees were there for only two things: either you would spend the afternoon in bed with them, or they would sort your mail. In 1963, she went undercover and trained to be a Playboy Bunny for Show magazine, revealing in the process just how badly the girls were treated ("I learned what it's like to be hung on a meat hook… the costume was so tight, it would have given a man a cleavage"). But though this eventually helped her to get work at the New York Times and New York magazine, she still found herself writing about such important matters as textured stockings. Did this make her angry? "Well, I was disappointed that this was what I was stuck with, but I did try to get absorbed. I researched the hell out of textured stockings! And I was happy to be making a living."

Then, overnight, everything changed. She went to cover an event – a "speak-out" – in which women would talk about their experiences of abortion. Steinem had had an abortion herself, aged 22, in London. But she had never spoken of it. She felt a "big click". The secrecy surrounding abortion suddenly seemed so oddly counterproductive. "It [abortion] is supposed to make us a bad person. But I must say, I never felt that. I used to sit and try and figure out how old the child would be, trying to make myself feel guilty. But I never could! I think the person who said: 'Honey, if men could get pregnant, abortion would be a sacrament' was right. Speaking for myself, I knew it was the first time I had taken responsibility for my own life. I wasn't going to let things happen to me. I was going to direct my life, and therefore it felt positive. But still, I didn't tell anyone. Because I knew that out there it wasn't [positive]." A low laugh. "I don't know about you, but I re-virginised myself several times, too."

Before she knew it, she was a fully paid-up member of the women's movement, and she regards it as having saved her life. "For me, this is, and always has been, politics 101," she says. "The idea that women are supposed to be the means of reproduction. If they – I mean 'they' in the larger sense: patriarchy, nationalism, whatever you want to call the mega-structure – didn't want to control reproduction, we wouldn't be in the mess we're in. Remember my age. I didn't know that I had a choice for a long time. I didn't want to get married and have children, but I thought it was inevitable, and so, I kept saying: not right now. I kept putting it off. After feminism, I suddenly realised: not everyone has to live the same way. Imagine that!" She never did have children – though she eventually married – and has never regretted it. "I suppose I could analyse it, in the sense that I looked after my mother. But I don't know that's really it. That's too neat. I just never wanted to." Her sister, on the other hand, had six. "Yeah, she took care of my social obligations. She once said to me: 'I'm really glad you didn't get married and have children. If you had, then you would have it all, and I would be jealous.' I thought that was very honest." Didn't she worry about being thought of as cold and unfeminine? She casts me a look. "Who wants to be feminine?"

Steinem never intended to be so visible a figure in the women's movement – or so she insists. She hated public speaking, and feared conflict. "I know. I'm in the wrong business. But you have no choice, however hard it is. I experience it like this: either I am invisible, or someone I identify with is invisible, and it makes me so angry. It's so wrong, and then I just can't resist. I have to do something." Is she tough? "That's a good question. I don't know. Different things hurt you surprisingly. But I always had the feeling, which makes you tough under duress, that I was a survivor."

But whether she intended it or not, there followed the most remarkable and radical few years – so radical, in fact, that when you look at the footage, you can hardly believe this was America. Steinem's critics like to point out that, though she has published several books, unlike Betty Friedan's The Feminine Mystique, or Germaine Greer's The Female Eunuch, none of them is a set text; her fame, they say, is disproportionate to her influence (and, boy, was she famous: Richard Nixon was recorded furiously ranting about her; his secretary of state, Henry Kissinger, meanwhile, once made a flirty reference to her in a speech). But no one can say that she didn't get stuff done. She led – in boots and polo neck – march after march. She testified in the Senate on behalf of the Equal Rights Amendment, and co-founded the Women's Action Alliance and the National Women's Political Caucus. The first issue of Ms, which was the first periodical ever to be created, owned and operated entirely by women and sold out in a week, contained a feature titled: "We have had abortions". It was signed by singer Judy Collins, tennis player Billie Jean King, and writers Susan Sontag, Grace Paley, Anais Nin and Nora Ephron (not all of these women had necessarily had abortions; the statement was inspired by those non-Jewish Danes who, during the second world war, wore yellow stars, daring the Nazis to arrest them too). A year later the Roe v Wade judgment was passed down, and abortion was effectively legalised.

"Of course, it has gone back," she says, now. "In this large country, 85% of counties have no abortion services. The clinics that do exist are still under threat. The so-called 'right to life groups' are less likely to firebomb – they got called terrorists, which was awkward – but they still picket and run false clinics and take photographs of women as they go in. One of our two main political parties is anti-abortion, and in some states, they have passed extraordinary legislation so that even those who fall pregnant as a result of incest or rape must hear lectures and see ultrasound pictures. South Dakota tried to pass a law saying that murdering an abortion clinic doctor would be self-defence. It is a struggle, all the time, and it always will be."

In the earliest days of this struggle, she was supported by other women – civil rights activist Florynce Kennedy; Congresswoman Bella Abzug – and she loved the warmth of their embrace. "It was like finding a family," she says. "As wildly different as we may be, and we certainly were, women share hopes and a certain vision of the world, and a certain shit detector. That was one of the greatest rewards." It makes her cross when people complain that groups of women can be bitchy. "Do women compete for the favours of men? Yes. They've spent 5,000 years competing. It [competition] is true of any subordinated group. But once you get a sense of possibilities and shared experience, it becomes the most powerful community. I see a form of it when I travel. I'll be walking through an airport, say, and my plane will be four hours late, and a woman cleaner will say: 'Here, take these magazines I've collected', or: 'When I'm tired, I sleep in the closet over there. Would you like to use it?' It's the same with the flight attendants. It's a floating community."

Did men's attitudes to her change after she became a feminist? Were relationships more difficult? "No, on the contrary. It attracted people. It's much worse if you're pretending [not to be a feminist]; then you attract the wrong person. At New York magazine, I was the only woman, and after I wrote the abortion article, my [male] friends there said to me: 'Don't get involved with these crazy women. You've worked hard to be taken seriously.' And I thought: they haven't a clue who I am, and it's my fault because I never told them. So I had better relationships afterwards. I mean, I need to think about which lovers came before, and which after, but I think we're all still friends. That's the test." She smiles. "I remember so well the intensity and jealousy and all that. I always tell younger women: don't worry, eventually all your old lovers will be your friends."

The only mistake she ever made, relationship-wise, was media magnate Mort Zuckerman, in the late 1980s. She had turned 50; her mother had recently died; she felt exhausted. "When you're depressed, nothing matters," she says. "When you're sad, everything matters. They're profoundly different states, and that was the only time in my life I felt depressed, and the only time in my life I had a relationship with somebody whose values I didn't share. Nobody understood why I went out with him. But he could dance, he was funny, and I was too fatigued to argue. It was pure fun. He was somebody I knew I would never have to look after."

Steinem is used to hostility and, frankly, regards it as an improvement on ridicule. Still, even she was amazed by the response when, in 2000, at the age of 66, she married David Bale, a South African-born entrepreneur and environmentalist and the father of actor Christian Bale. The fish had found her bicycle! "Some people understood," she says. "But most didn't. What was odd about it to me – and I realise this is arcane – is that we had spent 30 years changing the laws. The institution of marriage had changed out of all recognition." But in any case, the truth is that she didn't suddenly think: I want to marry this man. That would have been to deny all that had gone before: the other men she had loved, the things she believed so passionately. "Why would I do that? We wouldn't have got married if he hadn't needed a visa. We loved each other, we wanted to be together, but we wouldn't have legally married if it hadn't been that he still had a British passport. The other element was that my friend, Wilma [Mankiller, principal chief of the Cherokee Nation] offered us a Cherokee ceremony." And thank God they did marry, she says. Soon after, Bale was diagnosed with brain lymphoma; he died in 2003, aged 62. Their marriage meant he could be treated on her insurance, a fact that has only increased her support for same-sex marriage.

His death must have felt so unfair. "Well, that's true, and it was terrible, terrible, the two years of his illness and afterwards. But, looking back, though I don't believe anything to be pre-ordained… there was a purpose. His purpose in my life was, as my friends all said, to make me feel deeply, and to live in the present. Because I live in the future. My purpose was to give him something that he hadn't had. We discovered that he liked to travel with me, and after an event [a speech on campus], he would have 500 young women around him, all so relieved to see that you can be a feminist and have a relationship with a man – and he was so excited by it. At Smith [College], he would walk round campus saying: 'Which one of these young women is going to be president?' And then… I ushered him out of life. His kids were there, and they were wonderful, but a contemporary is a different thing. In a way, he taught me about dying."

Has she been able to continue living in the present? Not really. The future runs through her brain like ticker tape. This book, and the next (about native cultures). The speeches she will make. The campaigns she will join. "Only the women's movement will take up gun control," she says. "Because women are both the victims and the false excuse for keeping guns: [they say they're] to protect our women." But she remains – startlingly so, I think – an optimist. "Of course I think we need to get much angrier about childcare, about flexible working patterns. It's alarming to me that women are still encouraged to blame themselves. No one can do it all. If I had $5 for every time we've tried to kill off superwoman, I'd be very rich. But women are planning their lives, they have choices, and that didn't happen before, believe me. We thought our husbands and children would dictate everything." Even the Dominique Strauss-Kahn debacle is, for her, a sign of how things have shifted: "The maid knew she had rights. The DA plucked the future president of France off a plane. That's progress."

Is she disappointed by Obama? "Well, however disappointed and however angry anyone may be, our only opposition party is so completely taken over by crazy people that to vote for him is to vote for ourselves. He has to win." But how does she feel about him? "I sort of feel the same [as I did before]. He has a good heart and a good mind, and while I appreciate that he doesn't increase [crank up] the hostility level, that's also a problem. Unlike Hillary [Clinton], who spent eight years under fire from crazed rightwingers, he is not a fighter. That was always the good news, and the bad news. I worked for Hillary, and I thought she'd be the better president. But I never thought she'd win. It was too soon for a woman to win." But when will it not be too soon? She doesn't know. The dream would be another Obama term, followed by Clinton standing for the presidency. "Whatever happens, I think she has made a difference. She has presented a vision of a woman who could be president. She was so smart and so brave, and she hung in there. She's a miracle. She has changed the molecules in the air."

We have been talking for a long while now, and Steinem, almost in spite of herself, is finally giving me the whole of her attention. But then, a door opens – "That's my Kenyan house guest," she says – and the bubble has burst. I can sense her wanting to check her emails.

We head for the door. She hugs me, and I realise how slight her body is; she feels almost frail, which is a surprise. Then she hands me Morgan's book, which she has signed. It seems all of a piece with Steinem's generosity that she would give me another woman's book rather than one of her own (either that, or she sincerely believes I will find it useful to be able to quote screeds of Thomas Jefferson, Susan B Anthony and Toni Morrison at my enemies). On the subway, I open it and read what she has written. "To Rachel," it says. "Who is part of my/her own country!" I'm not exactly sure what this gnomic phrase means. But perhaps it's only the spirit of the thing that counts. As the train clatters downtown, I allow myself to feel feisty, and just a little bit fond.