BY THE TIME I ARRIVED at the June seminar, Gym Jones had just moved into its fourth and, Twight hoped, last location—a 6,000-square-foot garage full of black rubber floor mats, barbells, squat racks, and other torture-chamber accoutrements. As promised, the only mirror was in the bathroom, and the block walls featured just two decorations: an American flag and a sign, cribbed from Fight Club, that read in part, "Every word you read of this useless fine print is another second off your life. Don't you have other things to do? … Get out of your apartment. Meet a member of the opposite sex. Stop the excessive shopping and masturbation. Quit your job. Start a fight. Prove you're alive."

If I wasn't the weakest person there, I was pretty far down in the pecking order. The group included an Ironman triathlete from California; a couple of ski mountaineers from Boulder; a jujitsu fighter from Australia; and a New Orleans–based former SWAT commander with a full sleeve of tattoos. A few rows of metal chairs had been set up in front of a whiteboard. I sat in the back near several guys who looked to be more my speed: T.J., a recent college grad who'd come at his parents' urging; John, an engineer and aspiring marathoner from Ontario; and Andy, a paunchy father of two from New York who was trying to get back in shape. Twight, who looked like Sgt. Rock with his jutting chin and shorn, graying hair, passed out three-ring binders, along with a copy of Kiss or Kill. Promptly at nine, we dove in.

The whiteboard session lasted most of the morning, and I tried to absorb it all despite a persistent sense of dread about the looming workouts. At last we retired to the mats and warmed up. Our first circuit was called Tail Pipe. It involves rowing 250 meters on a Concept2 machine as fast as you can while a partner stands in front of you, holding two 53-pound kettlebells at the top of his chest until you finish. As soon as you're done rowing, you switch places until you've done both exercises three times, without a break. I teamed up with Andy, and we grunted through the drill in just over nine minutes. The fittest guys in the class finished in about half that time.

"Why do you call it Tail Pipe?" I asked one of the coaches, still doubled over and wheezing like an asthmatic.

"Because," he said chirpily, "it's like sucking air out of a car's tailpipe!"

The afternoon wasn't much better. I struggled to complete the Kettlebell Complex—a series of awkward lifts and swings—and dropped out of Nothing But Pull-Ups altogether. Sunday followed a similar program and, much to my dismay, got even harder, since it involved dead lifts. The dead lift is a straightforward exercise that involves standing in front of a loaded barbell, squatting down, and then hoisting the weight to your thighs. The more weight you add, the more it feels as if your spine is going to splinter like a celery stalk. Lifting twice one's body weight is considered a respectable benchmark. The world record is a little over 1,000 pounds.

I got up to 315, about 85 pounds shy of my double-body-weight fantasy, before throwing in the towel. I had been trading turns with John, the marathoner, who I thought had stopped because he was worried about an old back injury. But then I noticed him walking in circles in the corner of the gym, listening to his iPod, clenching and unclenching his fists. At 45 years old and 158 pounds, John was both the oldest and smallest guy in the class, and everyone gathered around as he fought all the way up to 325 pounds, dropping the bar with a theatrical grunt, followed by a round of cheers. I asked him later what he'd been listening to.

"The Strokes," he said. "I love music. It gets me so pumped. Before that last lift, I could feel the hairs on my neck standing up."

The day concluded with the Jones Crawl, a two-exercise ordeal in which you're supposed to dead-lift 115 percent of your body weight ten times, then complete 25 two-footed jumps onto a 24-inch-high box—three sets, as fast as you can. Even the tougher seminarians seemed nervous. As we took our places, one of them cried out, "Spartans, prepare for glory!"

I tried to do 200 pounds, but it was too heavy. I barely completed the required ten dead lifts in the first round and had to sit, huffing, before I could contemplate the box jumps. Twight allowed me to reduce the weight, but only slightly. By my second set, I could no longer speak. During my third round of box jumps, I caught my toe and wiped out, raking my shin against the box's wooden edge and removing a strip of flesh. I looked at Twight, who I assumed would let me quit, since a bright red rivulet was now running down my leg. "Bloodsport," by New Model Army, thundered from the boom box. "Last round," Twight said, expressionless. "Let's go."

The whole thing lasted about eight minutes, though it felt like eight days. The only person with a slower time than mine was T.J., the college kid, but he'd lifted more weight.

That night, I staggered back to my room, haunted by a December 2005 story in The New York Times called "Getting Fit, Even If It Kills You," which dwelled on rhabdomyolysis. "Rhabdo," as gym rats call it, is a condition brought on when muscle tissue is so severely damaged that it releases myoglobin and other harmful substances into the blood, triggering kidney failure. Rhabdo has been linked to high-voltage electric shock, car accidents, physical torture, drug abuse—and, in a few cases, high-intensity workouts.

I was OK, just whipped. Before I finally crawled into bed, I stood shivering in an ice-cold shower, because we were told it would help reduce muscle swelling.