Nicks is as warm and empathetic in person as her endless hit songs (Dreams, Rhiannon, Edge of Seventeen) suggest. She is relaxed in black, drapey loungewear and dark, tinted glasses. Her manner is a combination of floaty New Agey-ness and quick-witted humour. It is late July and Fleetwood Mac's 2013 world tour is on a short summer break. Nicks and her assistant, Karen, have duly retreated to the beach. From her panoramic windows she can watch the cars twinkling down the Pacific Coast Highway at night. Nicks also owns a 1930s "English-style" mansion in the nearby hills. Three years ago she recorded a solo album there, In Your Dreams, with the producer and former Eurythmic Dave Stewart. But this apartment, with its 180-degree views up and down Santa Monica Beach, is where Nicks comes to recharge. Fleetwood Mac have sold upwards of 100 million albums. When 1976's Rumours (which has sold 40 million copies) was re-released this year, it went back into the album charts at No. 3. Forty years at the top have made the band very rich - rich enough for Mick Fleetwood reportedly to have blown $8 million on cocaine over the years, and for Nicks to own multiple properties. This apartment used to be somebody's bachelor pad, and the high-tech entertainment system that the previous owner installed has been a bit of a problem.

Nicks, a self-confessed Luddite, only recently bought her first mobile phone. "I had to escape a fire in Malibu a couple of years ago. That's when I realised I had to have one. Now I do have a cellphone. It's not charged, but it's over there somewhere.'' She is even contemplating going online. "Just 'cause I don't like the internet doesn't mean that everybody's gonna stop using it." She pouts with a self-mocking air. "So I'm starting to think: You know what? Am I just punishing myself? I might as well know how to do it." Fleetwood Mac - Nicks, Lindsey Buckingham on guitar, John McVie on bass and Mick Fleetwood on drums (all but Christine McVie from the "classic" line-up) - have been on the road for months. "Two hours and 40 minutes, every night!" says Nicks with a gasp.

She is enjoying it now. "But in the beginning, before the shows, truly, I'd look at Lindsey as we were going up to the stage in the lift and go, 'This is too much for me.' And he'd go, 'What do you want me to say?' And I'd go, 'Well, I don't know, some sympathy would be good. Something like, "I know you're an old woman and this is hard on you - but you'll get through it!"'" Of course, Nicks knows Buckingham better than that. They met as teenagers in northern California in 1965. He was the high-school swimming champ and a prodigal guitarist; she was the girl from Arizona, relocated to the San Francisco area, thanks to her father's corporate executive job. They became friends, then bandmates, then, finally, a couple. As part of a four-piece band called Fritz, they opened for acts including Janis Joplin and Jimi Hendrix, who she remembers looking in her direction before announcing: "I'm dedicating a song to that girl over there." "That was a moment of greatness," she sighs. "I give him and Janis Joplin and Grace Slick [of Jefferson Airplane] the three nods. From Grace, I got her kinda slinkiness. Janis was just little with a big attitude with big hair and feathers, and a drop-dead amazing voice … And Jimi Hendrix, on the other side of that, was completely and utterly humble … So from those three people I got slinky, attitude and humility." Throw in the most directional deployment of scarves and chiffon in music history, "and that was kinda my stage performance," she says with a shrug. A rock'n'roll icon was born.

Having moved to Los Angeles, in 1974, Nicks and Buckingham joined the British blues band Fleetwood Mac, who were then living in the city. Famously, three long-term relationships ended during the making of Rumours, including Nicks and Buckingham's. But the band soldiered on through the heartache, the romantic turbulence helping to create some of the most emotional yet melodic and uplifting music ever. Nicks went on to have relationships with, among others, two members of The Eagles (Don Henley and Joe Walsh) and even had a fling with Mick Fleetwood. Buckingham is now married with three young children, but the spark between him and Nicks never went away, in ways good and bad. During the making of Fleetwood Mac's 2003 album Say You Will they had a falling-out from which they'd never fully recovered. So, prior to this year's tour, Nicks decided they needed to clear the air. "And I said to him: 'I believe that you and I need to remember who we were when we were 16 and 17 years old. We need to remember that we were really good friends before we ever had a date. We need to remember how much we respected each other, how much fun we had - and how much fun we can have when we're both in good spirits. And we need to take that power couple on to the stage. Or we need to not go on tour.'" Did Buckingham agree? "He listened to me, and I'm just sorry we didn't have this conversation 10 or even 20 years ago."

The heart-to-heart did its job. "We are having fun," she says, clearly delighted at the late-flowering harmony in this legendarily dysfunctional band. As Buckingham recently put it, describing the American leg of the tour, "It's a little bit of a love-fest up there." Nicks has legendary status as one of the first female rock stars. She sang with Taylor Swift at the 2010 Grammy Awards, an experience a besotted Swift described as "a fairytale". The appeal of Fleetwood Mac's music to a younger generation was underscored by a 2011 Glee episode devoted to Rumours. Nicks visited the show's Los Angeles set during filming, and grew close to the cast. After Cory Monteith, who played Finn Hudson, died of a heroin overdose in July, Nicks wrote the cast a letter. "I said, 'Forgive me if I'm stepping into your personal space. I lost my godson to an overdose last year - he died at a fraternity party - so I do understand how shocking that is.' When what happened to Cory happened, I just was sick for days. Honestly, just sick. It's just so prevalent. It's everywhere. It's worse than when we were young." That is saying something, given how notoriously hard-partying Fleetwood Mac were throughout most of the 1970s and 1980s. Nicks recalls they were warned of the dangers of heroin. "But I wish people had told us the same thing about cocaine. But in our day everybody was going, 'It's not addictive, it's just recreational fun, blah blah blah'."

The band's cocaine habit led to Nicks becoming addicted for 10 years. In 1986, after being prescribed the sedative clonazepam to wean her off cocaine, she became addicted to that. Her weight ballooned, her skin peeled, her hair fell out, she became a quasi-recluse. She lost eight years to clonazepam, and often wonders if she might have met someone and started a family in that time if she hadn't been so zonked out. "That was stolen," she says of the chance to find happiness in that period. "And that's not forgivable, ever." Nicks is content to be single now. "I think that I'm stronger than a lot of people," she says. "And I knew then, as I know now, that I still have so much to do." As well as completing the Fleetwood Mac world tour, Nicks is working on plans to release a long-lost pre-Mac album that she and Buckingham made as Buckingham Nicks. She's also an enthusiastic painter (of angels, mainly), a poet and a reader, and is hoping to realise a long-cherished dream: to turn The Mabinogion Tetralogy, four obscure books based on Welsh myths, into a mini-series. She's owned the rights to the novels for more than 30 years and is hopeful that the makers of her favourite television show, Game of Thrones, will be interested in turning them into a screenplay. The Telegraph, London

Fleetwood Mac play at the Entertainment Centre on November 10-11, and the Hope Estate Winery, Hunter Valley, on November 16.