Yesterday, I played football for the first time since the reign of Margaret Thatcher. I was staying with a friend who has a game in the local park every week, so I had to join in - and I can't remember the last time I was so utterly exhausted. I now understand how footballers can look so convincingly agonised when they take a dive. Fifteen minutes after kick-off I felt quite capable of rolling around on the ground in tears at a moment's notice.

This physical failure seemed particularly unfair because I started exercising months ago: I swim twice a week, and on those mornings when I don't have time to sit stationary on a broken Docklands Railway train for an hour, I walk to the Independent's office. And as luck would would have it, this turns out to be quite a twentysomething thing to do. One by one, in the past year or so, my friends have started signing up to gyms, seeing if their swimming trunks still fit, putting dumbbells on their birthday lists. Even my friend Douglas, who stood at the 18-mile mark of the London Marathon a month ago with a Marlboro in one hand and a can of Heineken in the other, shouting, "You're not feeling so bloody fit now, are you?" at passing runners - even he has been spotted on an exercise bike.

It's not that you have to do more exercise than ever before as a twentysomething; it's just that you have to make a conscious effort even to do as much as you used to do without thinking about it. As a general rule, recreation for children is based on exercise, while recreation for adults is based on consumption. Given a sunny weekend, a child will climb a tree; an adult will drive to a nice country pub for two pints of Caffrey's and a Cumberland sausage. And even students exercise without trying, just by walking and cycling. I'm strongly in favour of car adverts being tagged with a government health warning: prolonged use can make you fat.

It's a shock to discover that my dutiful bouts of self-punishment burn fewer calories than my strolls to the shops and back used to, but one has to learn from life's experiences. What I learnt from my game of football was that the idea of stretching before exercise isn't just silliness made up by aerobics instructors, as I'd always assumed, but is actually a sensible idea. I'll bear it in mind the next time I play football, an event which I'll be sure to write about in a column entitled 'postcard from a fortysomething'.