I'm a fitness addict. I have swum a kilometre nearly every day for the past 26 years, and for the past six have got up at 5.30am to do it. I swam until the day I went into labour with each of my three children, have driven friends and family mad by arranging my holidays around swimming, and have been known to plunge into icy seas, rivers and lakes all over the world rather than go for a day without being submerged (and no, lying in the bath won't do).

My fixation started in the days when pink velour tracksuits were fashionable the first time around and Jane Fonda was still better known for her opposition to the Vietnam war than for aerobics videos. In those days "lane swimming" didn't exist and my local pool in north London was populated by small children, a hard core of like-minded obsessives (some of whom are still close friends) and a small, formidable group of elderly mid-European women who swam furiously before adjourning for coffee in the long-gone Cosmos on Finchley Road. In spite of their years, they were brimming with vitality and vigour and, even at 19, I knew I wanted to be like them when I reached their age.

Since then, the fitness industry has exploded in this country. There are gyms in every neighbourhood; you can't move for lifestyle gurus telling you how to breathe; most of the old municipal baths have become leisure centres; offices spew out joggers at lunchtime; personal training is a recognised qualification; and Speedo has its own shop.

Even my other half [Alastair Campbell, Tony Blair's former communications director], whose irritation at my swimming used to know no bounds - especially in his days as a Daily Mirror reporter, when raising a pint and a fag to his lips was his only form of exercise - has now run the London Marathon in under four hours. In fact, our house sometimes resembles the Olympic village in the morning - my 16-year-old son regularly runs six miles before school in preparation for the medal he is sure he will win in 2012, the next one is a tennis fanatic, and my nine-year-old daughter, eager to please, swims, runs and plays tennis (although she sometimes tells me plaintively that all she really wants to do is be an actress).

So why do we do it? My son (when not using the word to describe the pupils of the nearby girls' school) tells me that "fit" is something everyone wants to be. Unfortunately, darling, that is not the case. The fitness boom has been accompanied by a depressing rise in the number of obese people, a new couch generation, early-onset diabetes and fat farms for children.

I once shared an office with a lovely woman who was on a permanent quest to lose weight and give up smoking. I lived through the cabbage diet, WeightWatchers, Atkins - you name it. She also paid a huge monthly sum to a gym she never attended. I finally prised the reason out of her: she couldn't face going until she had lost enough weight to give herself the confidence to appear in her workout gear in front of anyone else, or, indeed, herself in the mirror. It was a blunt reminder that huge barriers to exercise exist for many people - it's not just laziness, but also self-esteem. To her, my single-minded determination was intimidating, and keeping fit was for "other" types of people.

I am an evangelist on the subject, but for good reason. As well as the personal waste of people feeling lousy about themselves, there are huge public policy implications in raising a new generation of lazy, physically inactive people. We would be better off persuading a lot of people like my friend to become a bit more active than having a minority of super-fit nutters like me.

Keeping fit doesn't have to be scary. When I pressed my son on the meaning of fitness, he came up with his GCSE PE definition: "The ability to meet the demands of the environment." His alpha male father hotly disagrees and prefers "the ability of the body to reach its maximum in sporting endeavour". But, on reflection, the old textbook definition is pretty clever.

My daily swim has kept me sane, calm, healthy (and thin) through many trying professional and personal times. Its benefits are as much mental and emotional as physical. In my 25 minutes in the water, even at 6am, I problem-solve or think about nothing at all. The endorphins then take over and anything becomes bearable. I am rarely ill and have always been able to eat what I want without gaining weight.

Exercise makes you hungry, but also subconsciously changes your behaviour in other ways. Healthy food seems more sensible and you start questioning the value of "empty calories" at the same time as building the muscle that helps to burn them off. In my case, it was also responsible for me tossing my last packet of cigarettes in the bin 20 years ago - and diving into a freezing pool with a hangover at the crack of dawn is definitely a deterrent to drinking too much. When I told a friend I was thinking of writing something about keeping fit, she was aghast and told me I was completely wrong for "that sort of thing" because I was already thin and fit, so what could I prove?

Well, at 46, working from home again after a long period in an office, facing menopausal middle age and with an urge to travel to places without a handy patch of water, maybe it is time to face down my own compulsion and find something more portable than swimming. Could I become a runner, for example? Is yoga exercise? What exactly is spinning? And what do you do on those big round exercise balls that look like something children used to bounce down the road on 26 years ago?

· Fiona Millar's fitness column starts next week.