"My job was taking over my life. My former partner died. I had to be forced to take time off. It's not been an easy year"

At Brown's restaurant she appears relaxed and energised, ordering a fresh mint tea, which arrives swarming like a pond full of weed. "Looks like a salad, doesn't it?" she jokes. "That could be a classic rabbit lunch." Her health is at the centre of our conversation. What, for instance, was the health problem that caused her to take the time off sick?

"My health had been compromised by working too hard for too long," she says. "I worked myself into the ground with various adverse effects on the things you need, like your body and your brain."

There had, I suggest, been speculation among the Edinburgh arts community, that, given she had lost a lot of weight, she perhaps has an eating disorder. Since taking on the directorship she had shrunk from a plump ripeness to lean frailty. "No," she laughs. "I don't. I lost a lot of weight very quickly because I had to. I had become accidentally overweight, and so I lost weight." The weight, she describes, had accumulated, bit by bit, over the years. "Because I don't care about my appearance, it just sort of crept up."

It's not difficult to see how she careered into this health crisis. Since she left her position as literary editor of The Scotsman and took on the directorship, everything about the festival had expanded - the number of authors visiting, the team of staff, the square footage of tent space - and yet she had continued to carry its vast programming around inside her own head. She was getting through an average of a book a day, reading most of them at night when she would sleep a paltry four hours. She and her colleagues used to joke about the hypothetical "Number 27 bus scenario", a doomsday moment in which Lockerbie could be mown down by a passing vehicle and the entire festival plans lost.

Part of what makes the Edinburgh International Book Festival special is its egalitarian nature - all authors are paid the same flat fee regardless of profile and all get the same attention - and yet the only thing, Lockerbie notes, that was hierarchical about the whole event was her autocratic approach to programming. During the three weeks of the festival itself, she would make Charlotte Square Gardens her own private holding cell, spending 16 hours a day in there, and insisting on personally greeting each of the authors. The way she worked was "a function," she says, "of the amount of obsessive passion I put in."

In the end, she says, she had to be pushed to take the time off. By whom? "By a combination of people. Fundamentally by my doctor." And who else? Her colleagues? "It was a bit more complex than that," she says. However, it becomes clear during the course of our conversation that Lockerbie's festival colleagues did play a role in compelling her to take time off.

She professes not to have seen the signs herself that she was not coping, though, it is obvious that physically, she must have been rundown. "I didn't acknowledge it for a long time, which is why I eventually had to be pushed. Though I did make the decision myself to take time off in the end, it was with terrible bad grace and a lot of persuading."

Even when she did go off sick, Lockerbie was still working, conducting meetings and securing sponsorship deals like an expelled child who can't resist going back to the classroom. Days were spent reading books by authors coming to the festival. Work pervaded her leave. It's for this reason, whenever I ask her how long she was off, she is particularly hazy. "It's hard to say," she says. "It's been a bit of a struggle. There was a view that I should have been off, and I had a different view. I felt I should have been at work." Who held the opposing view? "A combination of people."

This breakdown was not, however, purely a result of her workload. It had been a tough year outside the office, in her life as single parent. Her son's father, Alain Monteux died just last Christmas, and his death strongly affected the 17-year-old Brendan Lockerbie Monteux. The fact that his father had not been present in his life since he was seven years old, Lockerbie and Monteaux having split up around that time, with Monteaux living in Istanbul, made it all the harder for young Brendan to deal with. "In a funny way it makes it more difficult for Brendan, because his dad wasn't there and now he'll never be there," she says. "It's been just a very tricky emotional territory. So, though I often say I have given my lifeblood and my passion to the book festival, obviously the primary lifeblood and passion has to go to my child. It's not been an easy year."

Lockerbie had met Monteux, the American grandson of Pierre Monteux - the conductor of the riot-provoking first performance in Paris of Stravinsky's The Rite of Spring - while a 21-year-old working at Camphill near Aberdeen, a community for children with special needs. She had gone there, fresh from graduating from Edinburgh University with a first in Philosophy and French, seeking an experience to take her out of her comfort zone. She fell for Monteux, who was then married, and they left Camphill and moved to Edinburgh together. They remained partners for 20 years, though they never married. Says Lockerbie: "I didn't feel the need to marry. I'm not bothered about that." She and Monteux made an idealistic and adventurous couple. They took up posts as teachers in Darfur together, which she says was where she "truly understood Islam for the first time". It seems to have been a feature of Lockerbie's younger years that she was a determined seeker out of experience. Even before Camphill, as a university graduate, rather than socialise with her peers, she hung out with writers 20 to 40 years her senior - "a bunch of reprobates" - including Norman McCaig and Sorley MacLean. The story of her life seems to be one of the girl who stands outside the crowd, often with her nose in a book. It's perhaps no surprise, given this, that for so long she carried so much of the festival like a lone warrior.

It was only on taking her current job that she began to deal with her weight problem. Any festival director, after all, becomes the much-photographed and much-filmed public face of the event, and must suddenly become acutely aware of his or her physical appearance. Yet, Lockerbie, has never seemed like someone who is particularly bothered about how she looks. She remains, she says, naturally scruffy and has made few compromises with her personal image. "I've never stopped being told off for my shoes," she sighs. "People say, Catherine, you've got to get a set of proper shoes.'" When she joined the festival one marketing consultant suggested, in the interests of getting her on television, she should be given a makeover. "I think not," says Lockerbie now, rolling her eyes. "I've never worn make-up. Ever. I just won't do that stuff."

Certainly, taking on the role has put her into a more public position than she might naturally have sought. She remains, after all, someone who "given all the choices in the world, would probably rather be up a tree with a book". Lockerbie describes herself as a shy person. "When I started out in journalism I used to have to interview people and I'd be terrified. I had to teach myself to engage properly with the world." As a child, she says, she was not particularly socially adept, and spent much of her time either reading or going on long solitary walks up hills near the family home at Bridge of Allan. Her father was a university lecturer and her mother a part-time teacher and she grew up in a house "with books in it". She was also often isolated by illness. "Oh it was great when I was little because I had every disease that was going: hepatitis, for instance, which was brilliant because it kept me off school for ages because my eyeballs were bright yellow. But I could read, and I read a lot."

Meeting her now, though, she seems to be blooming. Gone is that gauntness she seemed to carry at last year's festival. Her cheeks flush with a ruddy glow. I tell her she seems charged with energy. Does she feel healthy? "I feel determined. That's what I feel." Her intention is to go about this coming festival in a fresh manner. Rather than chaining herself to Charlotte Square Gardens, she has had the "terrifying notion" that she will accept an invitation to an "outside event". Her whole perspective too seems to have shifted. She is letting go of the idea that it is "her" festival. In conversation she is constantly amending and rewinding, censuring herself for having slipped out that word "my" again. "My programme," slips out accidentally, and she drops her head forwards to the table mocking an attempt to knock some sense into it. "Oh, stop it," she says to herself. "It's everybody's programme."

She is keen to credit other members of her team, including, in particular, administrative director Kath Mainland, who she says is "amazing". "I don't have a named deputy as such, but Kath runs the organisation whether I'm there or not. So I have to accept that I don't need to be there all those hours every single day, completely exhausting myself." In the past couple of years additional staff have also been hired to take on some of the load, including a programming manager, Roland Gulliver, and children and education programme director, Sarah Brady.

Is it, then, no longer so much her programme? There is a feeling this year that the line-up is not so strong because of her absence. "No," she says, "it hasn't suffered even when I've been off, I've been doing some work - and the team is brilliant."

She is, she says, often asked how much longer she will continue to direct the festival. "The honest answer is, I don't know. I think any festival needs, after a given period, new ideas and new energies, and that would normally mean a new director. I don't think any director should outstay his or her welcome." The important issue, she points out, is what the festival needs for the future. "But, personally, of course, you can imagine, from everything I've been saying, how hard handing it over will be for me. And what would I do next? I've got the best job in the world."

The festival approaches, and the question lingers of how she will survive it. She is, she says, intent on reconnecting with its pleasures. On her mind too is the celebration of her 50th birthday, which comes in its final week. "We've been talking about energy and a little bit about exhaustion as well," she says. "For me this role does both. It gives me all the energy and it exhausts me." One of the great moments of her directorship so far, she points out, involved the Albanian writer, Ismail Kadare. Asked why he thought he had survived the communist era, she recalls that he said, "the love of literature saved me". That line has a current, personal resonance for her. "I want the love of literature not to kill me," she says. "I want it to save me. I want to get the balance. That's my new mission."

The Edinburgh International Book Festival opens at Charlotte Square Gardens on Saturday