One weekend 10 years ago, a lawyer friend, Bill, got up at an uncharacteristically early hour and decided to go for a bike ride in Central Park. He was approaching Harlem Hill on his 10-speed when he heard it: a rhythmic, whirring rush, like a flight of locusts. Just like that, the peloton flashed by.

As any cycling fan knows, the peloton is the main cluster of riders in a bike race. There's that massive one you see in the Tour de France highlights every summer, snaking through the vineyards and fields of Provence, but the Central Park one can be pretty impressive, too — a helmeted, Lycra-encrusted horde of corporate-looking 30- and 40-somethings, with the occasional bike-messenger type mixed in.

Most people, when overtaken by the peloton, simply cower and get out of the way. But Bill had a different reaction. It was almost involuntary: His head dropped, his thigh muscles fired, and the spongy soles of his Hush Puppies dug into the corrugated metal points of his cheap pedals. It took an all-out 20-second sprint, but he closed the gap to the last rider in the pack and, as bike racers say, got on his wheel. And there he stayed for the next three laps of the park — almost 20 miles.

It was just a training ride, not a race, but afterward Bill, grinning like an idiot, tried to strike up conversation with some of his fellow riders. They ignored him, but he came back the next weekend, rode at the back of the pack again, and then summoned the courage to approach the guy who seemed to be in charge.

The coach eyed Bill doubtfully. "Well, first of all," he said, "you need to get yourself a real bike."

A month later, Bill was taking a bath when his girlfriend walked in. She knew something strange was afoot — a sleek Italian racing bike had recently taken up residence in the living room, and ever since then Bill had been rolling out of bed at 5:00 A.M. Still, nothing quite prepared her for the sight of her boyfriend stretched out in the tub, her pink Bic Lady razor in hand, methodically shaving his legs.

As we hurtle toward and past those frightening milestones — our fortieth birthday or, God forbid, the big 5-0 — it's always a comfort to joke about midlife crises, about trophy wives and Porsches as the cure for baldness. But when I step back and look at what my own little circle of friends and acquaintances is actually up to, I see only two categories of genuinely aberrant behavior. One, of course, is guitars — everybody who ever strummed three chords now owns a vintage Strat or a Les Paul, or both. And then there's the disease I call Ironman Syndrome — the mysterious, profound, and occasionally masochistic obsession with endurance sports.

There are plenty of aging sports nuts out there, of course — mad golfers, tennis bums, septuagenarian bowlers. But I'm not talking about a sport one actually plays. I'm talking about more primal forms of recreation: distance running and swimming, rowing, cycling, cross-country skiing, and, of course, triathlon. The sports that hurt.