When she was writing her first novel, Can We Still Be Friends, in which three young women struggle to make a life in 1980s London, Alexandra Shulman looked back on her diaries of the years in which the book is set. Most useful, she says, were not so much the "Oh God, I can't bear it …" confessionals, but her one-line engagement diaries, which reminded her where she had been and when. One repetitive line that cropped up in those volumes was "Lunch: Andrew Edmunds", and a corner table in the snug Soho institution is where we find ourselves talking about her book now.

You might have expected Shulman, Vogue editor for the past 20 years, to choose to lunch somewhere more ostentatiously cool than in the intimate, well-worn, candle-in-a-wine-bottle friendliness of the very-British bistro, but she is, like her surroundings, determinedly unpretentious and warm. She has been a regular on these slightly unforgiving wooden benches since the place was opened by Mr Edmunds, who owned the antique print shop next door. "I used to come first when I was working at the Tatler," she says. "[Satirist] Craig Brown would often come into the office around lunchtime in the hope that someone would be going out to lunch. And that was often me. So we would come here, have a bottle of wine or so, stagger back to the office at three, do a bit of work, Mark Boxer [the editor] would open a bottle of wine at six, then we would go off to a book launch party and that was that." Shulman lunches out less often these days, "maybe once a week," she says. "I like proper dinner at home, and I can't do both, and anyway I feel I've had enough nice lunches really."

She also has plenty on her plate. When we meet she is due to head off to the Paris couture shows in a couple of days, but before that has to finalise the programme for this year's Vogue festival in April, put some of the finishing touches to a one-off Miss Vogue magazine for younger readers, get some initial ideas together for the Vogue centenary in a couple of years time, as well as do the day job in which she now balances the traditional pressures of monthly deadlines with the daily demands of the magazine's website. "It's become much more journalistic," she says, "which I really enjoy, but which changes the pace a bit."

Shulman is the daughter of the late Milton Shulman, for 40 years the theatre critic of the Evening Standard, and she learned from her father the habits of getting things done without too much fuss. "I want to do things well but I'm not a total perfectionist. I think my dad was very much like that." Her mother, Drusilla Beyfus, who has written books about etiquette, and her sister, Nicola, are very much not like that, she says. "My sister wrote this amazing book, a biography of [16th-century poet and diplomat] Sir Thomas Wyatt, but like my mother she finds it hard to do things quickly; they agonise. I don't think I have the ability to agonise."

To prove the point she orders swiftly from the handwritten menu – cauliflower and cumin fritters to start ("I never order soup as a starter, since someone once told me 'you can't build a house on a lake'"), dressed crab to follow – and having firmly insisted she can't drink at lunchtimes these days, is immediately persuaded to join me in a glass of chablis, for old time's sake.

I wonder, given her commitments, what made her want to start writing fiction now, at 54. Did she want to prove something to herself? "I kept saying, maybe I will do it later, at another time in my life, and then I realised that after 50, later was becoming a more urgent sort of concept."

It was good, she says, to carve out a bit of space for herself. "I thought it would be good to think of myself as something other than the editor of Vogue. So I wrote at the weekends and got up early sometimes, five o'clock …" She has a son, Sam, from her marriage to journalist Paul Spike, which ended in divorce when she was 40. Sam is about to do his A-levels and she thinks that part of her was looking to have something to think about to replace him when he goes away next year. "I will have those early mornings to myself for the first time in ages," she says.

In looking back, the 80s came to seem like quite an innocent time. "There were obviously great hardships in the country, but for me it was an incredibly lucky time to be starting out in London. There was a feeling that there was suddenly tons of media, the international fashion industry here for the first time, Ralph Lauren and Armani, there was all this stuff going on."

Was the 80s a happy time for her? "I've been much happier in my 40s and 50s than I was in my 20s," she says – she has lived since her divorce with the writer David Jenkins. "I found those years difficult. I was always wanting something I didn't have. Not with work, but I was always having problems with boyfriends. I was renting flats for ages, and thinking 'how am I ever going to buy a flat?' I was much more introspective. I was forever wandering around Kensington Gardens singing sad songs to myself. Now I run around Kensington Gardens with power anthems on my iPod."

One of the striking things about the book is its portrayal of more robust sexual politics. Were offices more like battlegrounds for women? "Yes. Men now wouldn't dare do any of the things they did then. But I think I only benefited from, as it were, the sexual politics. Now I hear younger girls say: 'It's so creepy the way he puts his arm round my shoulder'. Well, to say that would never have occurred to me. All the men I worked with were lovely to me."

Young women these days, I suggest, have perhaps more to worry about from the pervasive imagery of perfection – and its discontents – that they are presented with in the media. Shulman has, not always completely consistently, tried to use her role to bang the drum against some of the more extreme aspects of body consciousness. "I have been going on forever about sample sizes, which is what we work from, needing to be bigger," she says, somewhat wearily. "We were photographing an actress the other day, very lovely, and only a size 10 or 12, but there is a very limited range of clothes you can find for her to wear. I think it is changing, but very slowly."

She is a staunch defender of the fashion industry in general, but believes it needs to be seen as fantasy. How do you make young girls see it that way, I ask.

She says she is excited by a current plan she has to put together "some kind of package" for schoolgirls, about what actually goes into a fashion shoot, "so the sense you have 24 hours to get the image you want, and 32 people working on it, and all the retouching and lighting and hair and makeup. Just to make it clear that this is not how anyone would expect to look when they get out of bed in the morning. It's a construct, like a movie is a construct. The harm is the idea they are reality."

Shulman is very much involved in that fantasy world, but likeably not really of it. She goes to all the fashion shows, and enjoys a lot of them. The various Vogue editors get together for a dinner in Paris each season. "It's more fun than you might imagine," she says. "Last year we had karaoke." She didn't sing, but has been threatened with the sack if she refuses this year.

I suppose in that event, I say, there is always a role in the diplomatic service to fall back on. Her opposite number in New York, Anna Wintour, has been widely tipped as Barack Obama's next ambassador to the UK. Does she think it likely she will also be getting the nod from our Foreign Office?

"Yes," she says, with a sly smile, "it's strongly rumoured in my office that I'm about to be offered Kazakhstan." OFM

Can We Still Be Friends is out now in paperback (Fig Tree, £7.99). To order a copy for £6.39, with free UK p&p, click here