I heard about Gym Jones the way you hear about a secret trout stream or an all-night rave—through word of mouth and friends of friends. To me, gyms tend to come in two varieties, neither particularly appealing. Option A: the Fitness Supercenters, with their overcrowded cardio dens and cucumber-scented saunas. And Option B: the Iron Grottoes, run by mulleted muscleheads in Metallica T-shirts. Gym Jones, the rumor went, was something else entirely—a new type of facility devoted to a mutant strain of fitness that combines elements of powerlifting, gymnastics, endurance sports, and military-style calisthenics. Insiders insisted that this odd hybrid was building bombproof athletes of all types and ages, and that the movement had acquired a devoted following in places from Washington, D.C., to Vancouver, B.C.

Gym Jones is located in Salt Lake City, where it was created by Mark Twight, a world-class alpinist who chiseled a career on walls of rock and ice so dangerous that, in some instances, he had only about a 50 percent chance of survival. Twight retired from climbing in 2001—at least from the suicidal stuff—and recommitted himself to the art and science of physical conditioning. His expertise attracted an elite cross section of clients—mountaineers, military special ops, cage fighters—but the gym, the origins of which date back to 2003, remained a largely underground phenomenon until 2007, when it was publicized that Twight had trained the British actors who portrayed Spartan soldiers in the war-porn fantasy film 300. Over the course of a few months, he'd turned the doughy troupe into a phalanx of freshly waxed Chippendales models, with marbled arms and abs like giant tortoiseshells. The superbods inspired a viral buzz that catapulted traffic on the Gym Jones Web site from a million hits a month to more than 11 million.

Like its owner, the gym took on a mystical, slightly nightmarish quality: part martial-arts dojo, part smash lab, part medieval dungeon, all intended to facilitate the arduous process of mental and physical transformation. When I checked out the Web site, I was greeted by a skull and crossbones and a warning: "Gym Jones is not a cozy place. There are no televisions, no machines, no comfortable spot to sit…. Effort and pain may not be avoided. Physical and psychological breakdowns occur."

OK, I thought, that which does not kill me … and then I e-mailed Twight, asking if I could come out to give it a try. His response was swift and dismissive. Who the hell was I? Did I have any idea what I was getting myself into? "Many of our guys worked out for a year before they started meaningful training," he wrote. "I'm not interested in having someone take a shallow look… . A quick peek is a waste of time."

I told him I was hoping for more than a quick peek. As a 41-year-old amateur athlete, I was acutely aware of the encroaching infirmities of middle age, but I hardly felt like my days on skis, a bike, or a soccer field were anywhere close to being over. I might be slipping, but I still felt strong and capable. With a little help, I figured, I could unleash whatever whup-ass was left.

Twight agreed to let me attend one of his two-day introductory seminars, provided I came prepared. That meant I had to "pretrain," so he put me in touch with one of his handpicked disciples, Carolyn Parker, a 39-year-old mountain guide based in Albuquerque, New Mexico, not far from my home.

Once a week for the next two months, I drove to a grassy field where Parker, a stunning physical specimen with a fondness for climbing waterfall ice in designer leather pants, schooled me in "the Gym Jones way." The first day, she threw a medicine ball so hard that it knocked the wind out of me. Other sessions involved dizzying combinations of dead lifts, squats, lunges, strange ballet-like kettlebell routines, and various other exercises of sadistic invention.

One day, we did something called "30-30s." Parker giggled maniacally as she handed me two 20-pound dumbbells. She told me to raise them to my shoulders and push them above my head as many times as I could in 30 seconds. Then I had to hold the weights up, arms locked, for 30 more seconds, and repeat until I'd finished four one-minute sets without a rest. By the last set, I could manage only a few reps, my arms noodling while I emitted a snorting sound, like a warthog. At the end, I fell into the grass, soaking wet. Parker stood next to me with her hands on her hips. "I think you're ready," she said.

And so, like some sad pilgrim seeking redemption through his own suffering, I arrived at an unmarked warehouse in Salt Lake City on a blistering June day, armed with just enough knowledge to stir a deeply nauseating realization: What I was about to experience was really, really going to hurt.