(FORTUNE Small Business) – I climb a sooty flight of stairs and open the wide, dimpled steel door to Gleason's Gym, a converted warehouse in the shadow of the Brooklyn Bridge. It's 7 A.M. on a Wednesday and barely 30 degrees outside. Inside there's the mingled odor of sweat, drying paint, and dried blood. In one corner, a few boxers pound jowly heavy bags. In another, a brawny trainer peddles a stationary bike that doesn't appear to have many miles of life left. Posters and sepia-stained clippings of past champions--Ali, Frazier, Tyson--wallpaper the 64-year-old gym.

This seems like the last place you'd find a radio tuned to Bloomberg Financial. But, of course, fighting isn't just for fighters anymore. In recent years boxing has become the workout of choice for those lunch-break spinners tired of going nowhere. And while the trendy version has been offered at upscale gyms for a while, a growing number of grittier (and more authentic) places are beginning to capture that clientele. Encouraged by this shift, I decided to act on my own long-held dreams of boxing.

Thanks to events like Gleason's monthly White Collar Sparring nights, non-pros of every shape, size, and sex are strapping on headgear, stepping into the ring, and trying their hands at full-contact fights. The Front Range Boxing Academy in Boulder, Colo., provides about 75 members with three weekly sparring classes. New York City's Church Street Boxing Gym recently began hosting regular White Collar Boxing shows, a more intense version of the Gleason's event, with pumping dance music and iron-pumped fighters. No longer content to just work the speed bag, men like John Oden also want to work someone over.

Oden, a principal of Bernstein Investment Research and Management, is a rakish fiftysomething with graying hair that brushes the tops of his ears. I sat with him recently, at a wobbly table at Gleason's, just a few sweat stains away from a couple of boxers jabbing at themselves in a long funhouse mirror. I listened as Oden spoke passionately about the sport he'd picked up while in his 40s. He joked that he fought more times last year than heavyweight champ Lennox Lewis did (Oden's five compared to Lewis's three). "I wasn't getting any younger," he says, explaining his decision to enter the ring. "And it was something I had always fantasized about doing."

Other would-be pugilists share the fantasy but shy from the reality. At Guard Up! Strategic Fitness in Waltham, Mass., members spar in what it calls "executive style," or with light contact. "A lot of the executives have to go to business meetings, and they don't want to go with two black eyes," explains Guard Up's Meghan Gardner, 32. "The biggest draw for many executives is learning skills they can apply to business," she says. "Like being flexible, outthinking their opponent, not forecasting their strikes."

For me, the fight fantasy isn't about business, but about romance. Stripped to its essence, boxing is the most honest job you can do. Gleason's is thick with the brutal, beautiful possibility of the sport. Several champions have trained here, including current International Boxing Federation junior welterweight champ Zab Judah. The first time I squeezed between the ropes and felt the surprising springiness of the canvas, I felt a bit unworthy of that remarkable history. I've often dreamed of being a boxer but always come up against one sizable hurdle: I don't care much for pain.

And nothing hurts like boxing. In the middle of my first lesson, on the verge of throwing up on my trainer, I bolted to a men's room stall and woozily contemplated the floor tiles. I couldn't open my locker; my arms were trembling too much (and that was just from a noncontact circuit). But that night I practiced punches in the mirror, my dead-eye stare coming into focus. If I could survive training, I began to think, I could take on anything or anybody.

At Gleason's monthly shows--now in their 12th year--teachers, judges, and money managers pay $20 to go three two-minute rounds with a well-matched foe. The events typically draw 100 to 200 spectators paying $10 admission. Owner Bruce Silverglade plays ring announcer and hands out tall, shiny trophies to all who compete. There is a referee, but no winner or loser.

At first the idea that no one can win frustrated my competitive urges. But the more times I saw fighters tangle with only pride and courage at stake, the more it made sense. Ultimately this sport is a competition with oneself.

The six-foot-four Oden trained for a year before his first fight. "I finally got in the ring one night with some big, burly, ugly guy," he says. "And he started crossing the ring [towards] me, looking at me with his guard up. And there I was. It was like a moment of truth: Am I really going to do this?" He did, but not before hyperventilating. Now he's a seasoned pug, looking to spar once more this year before hanging up the gloves for good. Says Oden: "People think of me as a much more interesting person because I box."

Others, like Mark Korman, are in it for the collisions. The stocky, 56-year-old Brooklynite is a tropical fruit buyer for supermarkets by trade but a boxer at heart. Once, moments before a bout, the Norman Mailer doppelganger announced, "I enjoy being able to hit and hit good." Sitting on the apron of the ring, around him fellow fighters taping hands and bouncing on toes, Korman leaned close to me. Cocking an ear toward a young fighter tenderizing a heavy bag nearby, he nudged me out of my romantic reverie. "Hear that noise?" he asked, grinning. "That sound excites me."

It's men like Korman who have delayed my sparring debut. A few times he playfully showed me some moves--and left my upper arms sore from half-speed blows. Though almost twice my age and blind in one eye, Korman has a true fighter's desire to vanquish his opponent. I don't know if I will ever fully develop that quality. But I intend to find out.