David Walliams: In at the deep end

Men approaching middle age tend to start setting themselves goals. Worried that the clock is ticking ever faster but full potential hasn't yet been realised, they embark on wildly demanding physical challenges or implausible construction projects. David Walliams' mid-life crisis is manifesting itself in a more ambitious manner than restoring a classic car or building a boat in the garden. Next week, just a fortnight after he celebrated his 40th birthday with a dinner at Scott's attended by, among others, Noel Gallagher, Gary Barlow and Natalie Imbruglia, the

It will be a long, exhausting, wretched, filthy and rather hazardous experience. During training sessions on the relatively civilised stretch of the Upper Thames near Marlow, Walliams' elegant stroke and bi-lateral breathing technique was regularly rudely interrupted. 'I was hit by rowing boat oars, attacked by swans and got covered in insect bites from all the bugs that live in the weeds,' he says. 'It is also very, very cold.'

The Upper Thames as far as Teddington is fairly swimmable, Walliams explains. 'But once you get into the tidal path, you get these really dangerous under-tows around the bridge supports.' And it gets even worse. 'If the rain is heavy, the river rises to above the height of the sewage pipes, letting raw sewage enter the Thames I'm praying it doesn't rain.'

Why swimming? It was the only sport he was ever any good at when he was a kid at Reigate Grammar in Surrey. As a teen he was, incredibly, the same weight as he is now - 'an overweight kid, basically, so when it came to picking sides at football I was always put in goal because they didn't want me running around the pitch.' He was also the only cub scout in the pack to fail his sporting badge.

Walliams can sometimes come across as intensely serious in interviews. He's polite, elegantly mannered and courteous, speaking evenly, in a rather actorly and measured baritone. He doesn't crack many gags. All the interviewer gets is an occasional Kenneth Williams-ish, mock seductive narrowing of the eyes and some bit of absolute filth delivered totally deadpan. 'I am built for swimming,' he explains. 'I am tall, I have long arms, big feet, big hands and, of course [pause for an archly comedic raise of the eyebrow here], a huge rudder.'

Considering his weight problems as a child it seems ironic that while training for a long-distance swimming challenge Walliams has to gorge on full-fat Cheddar and pastries from his local Primrose Hill delicatessen. 'Unless you are covered in a layer of flab - which is literally like having a wet suit on - you simply can't swim long distances in open water. You'll get too cold.'

He could, of course, avoid all the unpleasantness, humiliation, weight gain and time commitments and just make a big donation instead. I mean, what is he trying to prove by doing this? What would a psychiatrist say about it? 'He'd probably say it was a cry for help,' says Walliams, only half joking. 'I suppose I am thinking to myself, "What is going to bring me fulfilment in life?" And it's true that I go through periods of intense self-loathing. I think that's only human, isn't it?'

David Walliams' family was not affluent. His mother was a lab technician in a school, and his father, to whom he was less close and who died three years ago, was a Transport for London engineer. As a child, his sister used to dress him up in an old bridesmaid's dress and at school he was nicknamed Daphne and bullied for being effeminate. He studied drama at the University of Bristol and first met his comedy partner, Matt Lucas, at the National Youth Theatre in 1990. Together they slogged away for years; it wasn't until 2003, with Little Britain, their sketch show about the more idiosyncratic side of our national psyche, that they found real success. Just prior to this Walliams suffered a serious depressive episode and was unable to sleep for nine months until he sought professional help. He has been diagnosed as being manic depressive and has been in therapy.

Walliams admits that he likes - needs, perhaps - discipline and routine in his life. He thrives on the challenge of a long-distance event: the training, the diet, the abstinence, having something constructive and worthwhile to do; the sense of the self-confessed control-freak being in total control. He tells me a story to illustrate this. When he did the Channel swim in 2006, his nine months of training coincided with a huge Little Britain: Live tour of the UK. This was, remember, when the Vicky Pollard and 'Computer says, "No" ' characters were at their zeitgeisty peak, furnishing Lucas and Walliams with quasi-rockstar status. Paul McCartney came to a show. Michael Caine told Walliams that the duo was the new Monty Python. 'Many people in my position, comedians or musicians, might have taken advantage of the situation, going out after each show and getting drunk and high, trying to shag anything that moved. Instead, I'd go back to the hotel, have some herbal tea, go to bed, get up, swim for three or four hours, have lunch and then do another live show. It put me on an even keel.'

He saw how other modern comedians like to behave on tour when he went to a Mighty Boosh after-party in Dublin, 'Noel Fielding was in a nightclub jumping up and down on stage to Iggy Pop anthem 'Lust for Life', kissing all these girls who were throwing themselves at him. I stood there, watched and thought to myself, "God, I am so boring." '

We are having lunch at the Gilbert Scott restaurant in the newly refurbished St Pancras Renaissance Hotel. Walliams, as always, is immaculate in Tom Ford. 'I'm big,' he explains. 'You have to dress neatly when you're my size, otherwise you look like a big mess.' He reckons he caught his obsession with neatness from his mother, who used to pick single stray crumbs off the carpet with her fingers and hover around the bathroom door every time someone used the loo to check for water splash marks on the mirror above the basin when they vacated.