Three months ago Jessica Ennis won Olympic gold, confirming her status as both the friendly face of the Games and a ruthless winner. She talks about her split personality – and her extraordinary life since

After trying and failing to find Jessica Ennis in the English Institute of Sport in Sheffield, a female athlete helps me track her down. "There she is," she says. Which is just as well as I'm not entirely convinced I'd have recognised the slight figure in tracksuit bottoms and a hooded top.

I'm 5ft 10in, Ennis is just 5ft 5in, and with her muscles covered up and her six-pack under wraps, she seems teeny tiny, all skinny shoulders and narrow hips. If I hadn't seen her in action with my own eyes, and I didn't know she was the Olympic heptathlon champion, a sport which essentially involves being world class in seven different disciplines, I honestly would have said that I could have had her in an arm-wrestling match.

One of her trainers, Mick Thompson, recalls seeing her at an athletics open day which her parents had taken her to age 10, and says that, "She stood out a mile… she was probably one of most talented youngsters I've ever seen." We've become used to seeing these amazing physical freaks, the Michael Phelpses and Serena Williamses of the world, but Jessica Ennis just seems to be a perfect distillation of pure athleticism and absolute willpower.

In her about-to-be-published autobiography, Unbelievable, she recounts how before her first event in the Olympics, her mum sent her "usual" text message: "Don't let those big girls push you around." They didn't, of course. She didn't let them. Over the course of two days, and seven events, Ennis beat off the big girls and took gold in the Olympic stadium in a moment of high emotion in the middle of what came to be known as Super Saturday.

Two and a half months on, and, she says, "It still hasn't worn off." Has she had post-Olympic blues? "People ask me that," she says. "They say, isn't it an anti-climax? But it's been the best experience ever. And I'm still so busy trying to fit everything in. But it's still amazing. Every time my dad comes around, he's, like: 'You're an Olympic champion!' Everyone is still on a high."

But, she says, it's "weird" when she goes out. "You can hear people talking about you. And sometimes they see me and just go: 'Jessica Ennis!' like that in my face and then don't know what to say. But everyone is so sweet. They all just want to tell me their story. 'I was there!' Or 'I watched you and you made me cry.' That's one I've heard a few times."

Jessica Ennis wasn't just carrying her own hopes and dreams into the Olympic stadium, she had the wonderful but terrible psychic burden of carrying all of the rest of ours, too, because spontaneously and without actually asking her, the media, sponsors and Olympic organisers collectively decided that she was the face of the Games.

"I don't really know how that happened," she says. "One journalist asked me, 'Did you apply for that?' Like it'd been advertised or something. But, of course, I didn't. I really don't know why. It just sort of happened."

But everyone else knows exactly why. Her Olympic performance in the heptathlon was nothing short of spectacular. But it was her post-Olympic performance, that lovely genuine smile she has and the absolute heartfelt joy she showed in winning that cemented her reputation as one of the nicest people in sport.

But the pressure! In retrospect, and having read her autobiography, I'm amazed she didn't simply crack up. Or flee to the Priory. Or develop a cocaine habit. It's no coincidence, it seems to me, that in the ladies' loo at the Institute of Sport there's a poster with a tense-looking woman on it. "Feeling under pressure?" it asks. "Ring the Samaritans."

In Sheffield alone, she would pass five massive banner posters of herself on her way to training. She recalls how she once went out for a bag of chips and thought better of it when she saw herself blown up 20 times lifesize above the chip shop, the acme of physical perfection, and "drove on to Sainsbury's instead". The first thing that athletes arriving into Heathrow saw was a field-sized portrait of her face next to the runway. And arriving in Stratford, one athlete tweeted that it was like landing "in a Jessica Ennis theme park".

What were we thinking? It seems to me that if you were trying to make someone crack under the strain, that's roughly the way you'd go about doing it. "I'm not going to lie," she says. "This year has been really hard. I've always said that you're obviously doing something right if you have got pressure on you. But it was hard. It's difficult when you're in the thick of it, to fully comprehend, but when you step out and look back, I wouldn't want to do it again. Not unless I knew the outcome. It was the not knowing what was going to happen."

It's easy to think now that somehow it was all pre-ordained, but four years ago her training was going swimmingly and she was on track for the Beijing Olympics, when in a matter of seconds an injury changed all that. It was the darkest point of her career, though. "It changed me as an athlete," she says, and has made her success now all the sweeter.

The attention had its bonuses, of course: photo-shoots in LA with David Beckham and a 5-litre Jaguar from one of her sponsors. She's still only 26 and yet she's handled everything – the pressure, the nerves, the stress, and her glorious success – with great poise and grace.

And now there's Unbelievable, her memoir. In it, she describes how being small hasn't just defined her physically, but also mentally. It opens with an account of how she was bullied at junior school. "I was just different. I was such a scraggy little thing. And there were these two big girls who just used to pick on me." And, how, even now, going into competition against bigger, stronger athletes, the memory of that still spurs her on.

I can't help feeling that it's hammed up a little in the book. Ennis's autobiography was ghost-written by Rick Broadbent, a sportswriter on the Times, and while the bullying story forms a neat narrative to begin the book, I'm not entirely convinced of its significance. Given the lack of psychodrama in Ennis's life – she's close to her parents and her sister, had loving and supportive grandparents, her only experience of racism was one lone, not terribly traumatic incident at school, and she has been in the same long-term relationship for the past seven years – it has a slight clutching-at-straws quality.

The one thing that became apparent about Ennis during the Games was how normal she is. How humble she's been about her success and how grounded she is, still living in Sheffield where she grew up. But she's also, as she describes herself, "a competitive animal". And you just cannot underestimate the drive, determination and sheer dogged hard work that has gone into her career.

"Everyone else drifted away," she says. "Every year, I'd be training with a different group, because people didn't stick at it." And yet, Ennis did. She can't quite explain why. Her mother is quite competitive, she says. And her father did a bit of sprinting in Jamaica – where he was born – as a boy. "But then again, my sister absolutely hates sport and she was brought up exactly the same as me."

Because Ennis is so likable and easy going, it's easy to forget this essential fact about her: the steel that enabled her, after two days of competition, physically exhausted, facing her worst and least-favourite discipline – the 800m – and knowing that unless she fell over she'd done enough to win the gold medal, to still find the drive to overtake the naturally faster runners ahead of her.

"I know. It is a contradiction," she says. "I think that's one of the first things you'd say if you met me, that I am just nice and smiley. And that's how I might appear when I'm not competing. But I'm totally different when it comes to sport. It's just something that seems to be within me. It's not external or visual. But it's within me. You have to be totally up for it and motivated otherwise you just wouldn't win. But it's not something that a lot of people see. Apart from maybe my family."

There's a tiny hint of this on YouTube in a video that features an interview with her mother in which she says that, for a time, she thought that Jessica was "selfish" because her training – a strict, six-day-a-week schedule – and her needs took priority over everything else. "I know I saw that. I was, like, 'Mum!' But she's right. I admit it. I do have to be quite a selfish person because of what I do. You simply have to be. It's an individual sport and you have to be constantly looking at what's best for you. And everything has to fit in with where you want to get to. I think that changes through life, but when you're in the thick of it, you do have to be like that."

Ennis studied psychology at university, and even wrote her dissertation on "self- regulation", or self-discipline, and yet there's a sense that she is in some ways a mystery to herself. "I've always known what I wanted. I've always wanted to be on top of that podium." But why or how she's become the master of "self-regulation" that she is is as unknowable to herself, it seems, as to anybody. In her memoir, she says she tried basketball, but team sports weren't for her. She always wanted the responsibility of controlling her own destiny.

The cruelty of sport is that, in the end, however hard you try it's not entirely down to you. In the book, Ennis mentions Phyllis Agbo. They competed as juniors against each other, Agbo consistently beating her, and the one "who everyone thought would go on to do great things". I Google Agbo and find she suffered injury after injury and never made the squad.

"It's such a fine line. I know Phyllis really well. And she was the one who everyone thought would make it. But different things happen. And, as hard as you work, you still need luck. Before the Olympics I was in the best shape of my life, but I really, really needed some luck. And I got it."

ENNIS shares another trait with many successful athletes: a close and supportive family. Her mother is from Derbyshire farming stock and met her Jamaican-born father at the age of 19. He was 14 years older than her, and, just a few months later, she found herself pregnant with Jessica. "It wasn't very easy for them. They never had a lot of money."

Ennis started going out with her fiancé, Andy, a construction manager, at 19 – he was three years ahead of her at school – and he is, she says, "very understanding". He'd have to be, given her schedule, and the Denis Thatcher, fading-into-the-background nature of his role. But that's been her other luck. "He's been a really big part of my success. At the end of the day, I come home to him."

And despite offers of scholarships in America, Ennis has remained true and constant to Sheffield. One of the most satisfying incidents in the book is where the head of UK Athletics, Charles van Commenee, tries to lure her, and then force her, down to London. "I wasn't having it. I just knew it wasn't the right thing for me."

It's a rare glimpse of the other side of her. The resolute, take-no-prisoners, not-backing-down-no-way-never side. "I dislike conflict, so I do try and steer clear of arguments," she says. "Apart from with my coach. Who I literally have an argument with every day."

Ah yes, Toni Minichiello or Chell, as she calls him. She's been with him since she was 13 and their relationship is one of constant feuds and battles. Despite the fact that she studied psychology, she says she's no great fan of sports psychology, or "stating the obvious" as she calls it, but at one point she and Chell took themselves for what sounds like couples counselling. "We just had to learn how to communicate better. We are like an old married couple, basically."

And, here they are, together again. The Olympics was everything Ennis ever dreamed of, and more. And going back into training "has been hard. I can't deny it. But I'm five weeks in and it's gradually getting easier. I can feel my fitness returning. It was just such a big year last year. And I just wanted to make sure that I did everything right. And this year, the Olympics has gone, and I've won! And now I've got to get back into training, and I do think about Rio, but I can't think about it seriously yet, it's just too soon."

Next year will bring the world championships – as well as her wedding. Andy proposed two years ago, but all plans were shelved until after the Olympics. She wants a family, "and I don't want to leave it too late," she says, but "I've also got more I want to achieve". She says she just thinks she'll know when the time is right to stop.

"Men can go on as long as they like and still have a nice set-up at home. It is harder for women. But then that's true whatever career you choose. You have to make a choice. If you go away from your career you come back knowing you might be a lower level. That's a choice that each woman has to make. For me it's been so important to achieve what I set out to achieve first."

The Olympics was, however, a great boost for women in sport. Ennis is a natural diplomat, but she says that "a lot of people noticed the inequality with things like the Sports Personality of the Year awards last year when there wasn't a single woman nominated". And while it's immensely cheering to learn that she beat "Kim Kardashian and Kelly Brook" to win the title of "most desirable celebrity body", less cheering is the fact that if you put her name into YouTube there's no end of really quite pervy stuff out there.

She giggles when I mention it. "I find it quite funny," she says. What about 'Jessica Ennis and her perfect bum tribute', I ask. It's the first video that comes up. "Ugh. I've actually seen that," she says. "It's all slow motion. That freaked me out a little bit." It's not surprising. The first comment beneath the video says: "This is basically just a very strange and creepy video."

But if you scratch the surface, you'll find salacious montages of women athlete's bodies all over YouTube. It's incredibly depressing in all sorts of ways. Sport should surely be the one area of public life where women's bodies are judged for what they can do, not what they look like – but Ennis is sanguine. "I think it happens in every walk of life, doesn't it? None of us have gone into sport because we want to be looked at like that. We wear very little, but it's not for that reason. It's for performance reasons. It's a bit of a strange one."

Success has mostly been strange-good for Ennis, rather than strange-strange. She's almost universally loved. The nation's newest sweetheart. It's not difficult to see why she became the poster girl for our British 21st-century Olympics: hard-working but good-humoured, humble but determined, the smiling multicultural embodiment of how we'd like to see ourself as a nation. And when, despite the pressure, she came good, on Super Saturday, it's not a huge stretch to see that somehow, we did, too.

Unbelievable by Jessica Ennis is published on 8 November by Hodder & Stoughton at £20. To order a copy for £16, with free UK p&p, go to guardian/bookshop or telephone 0330 333 6846