The thought struck me as, for the third time in a week, I locked up my bike at work still panting, jelly-legged and bathed in sweat: aren't I getting a bit old for this? I've just turned 40 and it's not that my ride is especially long - a shade less than four miles - or strenuous. The trouble is that I can't resist an impromptu, unofficial race along the way.

It's been the same more or less everywhere I've lived, although London is by far the most competitive. Contests against fellow cyclists can take many forms, from a brief sprint between sets of traffic lights to a lung-bursting grind all the way to the office. The only prize, of course, is a slight glow of self-satisfaction, especially if I have just breezed past someone riding an expensive racing bike or clad head-to-toe in Lycra - better still, both. And nonchalance is the key. Neither party must acknowledge we are trying, let alone racing: after recreating a Tour de France sprint finish and reaching a red light, eye contact is carefully avoided, laboured breathing stifled.

My opponents take many forms, although, needless to say, they are invariably men. It's not only road racers or couriers who take offence when overtaken. Many times I've heard the muted panting and whirring chain of a cyclist approaching fast from behind and turned to see a suited businessman in fierce pursuit, briefcase strapped to the luggage rack, pinstriped legs pumping away.

Talking to other bike commuters, it seems I'm not alone. One friend described how easily he succumbs to temptation: "It's totally a matter of pride. If a Lycra-clad whippet on a flashier machine streaks ahead, I'm content just to try to stay on his or her tail. But if someone wheezes past on a rusting shopper - or worse, a folding bicycle - it's like a red rag and I'll show them who is king of the road."

This can, of course, be a lot of fun, but I'm increasingly aware of the drawbacks. For one thing, the physical effort involved means I'm committed to a shower and change of clothes when I get into work. And while I try to be a safe and courteous rider - I'll never pursue someone jumping red lights or weaving too hazardously through the traffic - I'm aware that not everyone sees it as fun.

"London's cycling culture can be a bit macho and it's sometimes not very inclusive for women," says a female friend who cycles merely to get from one place to another. "In other cities I've lived in, cycling has definitely been a more pleasant experience."

But another thought dominates. As I drift inevitably into middle age, isn't this all a bit undignified? Not to mention ever more difficult - there is only so long I can persuade myself that London's other cyclists are getting quicker, rather than me slowing down. I'm also arguably at that point in life when tight-fitting Lycra shorts become a liability.

So should I retire from these races? I could trade in my skinny-tyred, harsh-framed bike for a sensible, sturdy contraption with upright handlebars, full mudguards and a chain protector, even a nice basket for my bag at the front. Thus kitted out, rather than always pondering logistical questions of showers and fresh clothes, I could trundle gracefully to the office and arrive with nothing more than helmet-hair to contend with. Any time lost from a leisurely riding pace would be recouped elsewhere.

And yet ... I'm not sure I'm quite ready.

A few dry runs of this new-style journey while dressed in work clothes ended in sweaty failure after the stream of overtaking cyclists proved too much of a provocation. I'm also aware that commuting makes up the bulk of my cycling miles and it seems a shame to deny myself the thrill that comes from riding fast.

But above all, for much of the time riding to work is the only exercise I get, so I feel I might as well get the most from it. A stop-start London commute can seem a curious health regime but it fits surprisingly well with one approach to physical fitness: high-intensity interval training. This argues that a series of brief bursts of all-out sprinting, each interspersed with a slightly longer period of rest, can be far more effective than, say, half an hour of lesser, unbroken effort. By happy coincidence, riding full-pelt between London's many red lights provides just such a routine, at least according to my unscientific theory.

So maybe the wicker basket will have to wait for now. But that doesn't mean I am in complete denial about the advancing years. Later this week I plan to buy my first pair of baggy bike shorts; the Lycra versions will be going into retirement.