I STEP INSIDE the cage, or octagon, or what my mother once called "that prison-looking thing," on the floor of the Albuquerque Convention Center. I hear the cheers of 2,500 mixed-martial-arts (MMA) fans, though the stage lights make it impossible to see beyond the black chain-link fencing that surrounds me. They've come for FightWorld MMA 16 International, a night of 13 amateur and professional bouts. I'm in the opening fight. A referee wearing black plastic gloves gives instructions in my ear, but I can't make them out. He steps to the center of the mat and, with no ceremony, yells "Fight!"

I'd always wondered—and I know I'm not alone in this—how I would handle myself in a fight. The closest I'd ever come was a heated game of king of the hill in sixth grade. As an adult, would I?have the balls to repeatedly punch another man in the face or, perhaps more important, be on the receiving end of the same?

Last spring, I explained my ambitions to Tom Vaughn, co-owner of FIT NHB, an MMA gym in Albuquerque, New Mexico, near my home in Santa Fe.

I was 28, moderately active—I hike most weekends—but a decade removed from my peak fitness (high school football) and carrying only that 0–1 fight record from sixth grade. "We'll need to do some work," Tom said, looking me over. But he agreed to train me.

Our first session began with simple chokes and striking, then Tom demonstrated a defensive counter called "shrimping." To practice it, he had me lie on my back and push off with one leg while simultaneously turning on my side. The goal is to recover "guard," a neutral position in which one fighter has his legs wrapped around the other's waist. But with no one on top of me, I flopped around like a fish on the bottom of a boat. After a few minutes, Tom mercifully stopped me. The next day, his wife, Arlene, who is the gym's Thai-kickboxing instructor, had me practice basic kicks on the heavy bags for an hour. I felt on the verge of puking several times.

Eventually, though, I settled into a routine. Three days a week I drove the 60 miles to Albuquerque and began with an hourlong kickboxing class that often left me bruised and bloodied. The final 15 minutes were reserved for conditioning— wheelbarrows, squat-hops, and push-ups. Always push-ups. Next, grappling, which became my preferred style. (MMA fighters are generally either strikers or grapplers.) Tom would demonstrate a move, like an arm bar, heel hook, or triangle choke—each one a submission move that forces an opponent to surrender before a bone snaps or he is choked out—and then I'd practice with a partner. By the end of each back-to-back session, I was spent. After one particularly brutal day, my biceps were so sore that I had to brace one arm with the other just to brush my teeth.

But I was improving. After three months, I'd lost 15 pounds and was holding my own in sparring sessions. I grew more confident. Friends joked about starting bar fights so I could practice. Then one day at the gym, Tom pulled me aside. "I got a fight for you," he said, "in two months."

That had been my goal from the beginning, but suddenly, with the prospect of someone training specifically to kick my ass, it became visceral—Holy crap, I'm really stepping into that prison-looking thing. I immediately ramped up my training. Five days a week in Albuquer­que. Runs in the mountains. Hundred-yard sprints on a football field. Mock fights at the gym. There are plenty of motivators if you need a reason to work out: physical fitness, ski season, vanity. But nothing focuses you like the knowledge that your survival could actually depend on it. By the time fight night arrived, I was in the best shape of my life and, at 167 pounds, nearly 25 pounds lighter than when I'd started.

I met my opponent for the first time at the weigh-in, at a chicken-wing restaurant the day before the fight. Alex Gumaer, 24, from Oregon, was a first-time fighter too, with a chin-strip goatee and a reddish topknot that made him look like an Irish gremlin. Absurdly, I wished him good luck.

From that point until the start of the fight, a combination of nerves and adrenaline short- circuited my body. I was scared, but more than anything I was excited. Most fighters train for a year or more before stepping into the cage; I'd had half that. My modest goal: survive, and maybe make it past round one.

The first punch hits me with a vicious thud, just one second into the round. I hear it, and the crowd's reaction, but I don't feel a thing. Blood is pouring from my nose, and my eye is swelling up. But I recover and manage to score a takedown. We spend the rest of the three-minute round tied up on the ground, me in the dominant top position, scoring hits to Gumaer's ribs. Round two unfolds almost identically, minus the hit to my eye, and by the start of the third I'm ahead on the scorecards. We go to the floor again. But Gumaer gets top position and starts swinging wildly at my head. The first two rounds seemed to be over in a matter of seconds. But now everything slows down. I block with my forearms and try to shrimp my way to safety. I?hear the ref yelling, "Get out of it! Get out of it!" But I'm caught against the cage; no room to maneuver. And 34 seconds into round three, he calls the fight.

After the official announcement, I step out of the cage, onto the concrete floor, and see the crowd for the first time. Tom is next to me, his normally intense eyes betraying, I think, a sense of pride. Arlene wipes my face with a towel. "Nice work," she says, and gives me a quick hug. I feel a strange satisfaction rising in me. It's the first emotion I've felt all day. In the locker room afterwards, a doctor comes in to check on me.

"You all right?" he asks, feeling my bruised and bloody cheekbone for fractures.

"Never better," I say.

I mean it.