MY ARMS AND LEGS were tingling, and black vines were creeping into my peripheral vision. When I felt certain I was going to projectile-vomit on the monitor in front of me, I hopped my feet to the sides of the whirring treadmill and collapsed against the rail.

"Dark in there, isn't it?" said Neal Henderson, sports-science manager at Colorado's Boulder Center for Sports Medicine. Henderson was alluding to one of his favorite metaphors, the pain cave, from which I had just emerged after my seven-minute VO₂-max test. "That was good work," he continued, leaning forward to study the data displayed onscreen. "I think this is going to be a great result."

It was a cold, snowy day in January, and I'd returned to BCSM after five months of formal training to see what kind of progress I'd made. Henderson, a former nationally ranked triathlete and currently a certified strength-and-conditioning specialist, had been coaching me since September via daily e-mail prescriptions of running, cycling, weight lifting, and rest. (The latter, at times, seemed the only facet of my workouts at which I truly excelled.)

But even while micromanaging my day-to-day training, I knew the real secret to long-term fitness lay elsewhere: the calendar. Mine was currently marked with a series of events, starting in February with the Mt. Taylor Winter Quadrathlon—a punishing 40-plus-mile multisport race that involves cycling, running, skiing, and, finally, snowshoeing nearly 5,000 vertical feet to the 11,301-foot peak near Grants, New Mexico, then reversing the stages all the way back. Next was March's Elk Mountains Grand Traverse, a 40-mile backcountry ski race in Colorado, from Crested Butte to Aspen. Then in April I was heading off to Mount Everest, where I intended to climb to the North Col, elevation 23,000 feet, more than 5,000 feet higher than I'd ever been before.

It was an anxiety-inducing schedule, to say the least, exacerbated by the fact that I'd turned 40 and had spent many recent nights lying in bed, fingers laced behind my head, pondering some of life's deepest questions. Was I too old to date Scarlett Johansson? Should I abandon my dream of playing professional soccer in England? Was it still cool to wear my sleeveless Black Sabbath/Blue #Ouml;yster Cult World Tour '80 T-shirt?

Statistically, turning 40 plunked me almost squarely in the middle of the average American male life span, and the research I'd seen on this turning point was mildly troubling: Resting metabolic rate slows down, weight goes up, shiny sports cars appear in the garage. I kept thinking of that old Chinese proverb, "The sun at noon is the sun declining." But, athletically, had I really reached high noon?

I mentioned my concerns to Henderson, but he was more circumspect. "A lot of available data represents the way things are, not the way things necessarily need to be," he said. "Think of training like a savings account with compounding interest. The difference is that, with your body, when you stop investing, you also start automatic withdrawal."

This process, called "detraining," affects the lifelong ebb and flow of fitness. The bad news is that at any age, you detrain, or lose fitness, about twice as fast as you gain it—i.e., it takes two weeks on to make up for one week off. The good news, however, is that fitness gains can continue well into your seventies and beyond—and the sooner you start investing in your body, the bigger the dividends you can expect. Henderson showed me data on a 70-year-old runner who'd seen a whopping 20 percent improvement in his lactate threshold after about six months of training. So if forever young wasn't in the cards, maybe forever fit was within reason.

The results from my own recent BCSM test were frustratingly ambiguous. Since my initial test in September, my VO₂ max (a measurement of the oxygen your muscles can process) had climbed from 50.1 in September to nearly 54 in January—a respectable gain. But my LT, or lactate threshold—the point where your muscles produce lactic acid faster than they can process it, and perhaps the best indicator of endurance—was identical, right down to the decimal point. This wasn't entirely surprising. My training had been fraught with setbacks, including a shoulder injury during a December ski trip that had knocked me out of doing much of anything for nearly a month. As Henderson and I looked over my data, I wondered aloud if my VO₂-max result was just an aberration, an example of mind over matter and thus less promising than it appeared, but he was reassuring. "You have to be either more capable of suffering or more willing to suffer," he said. "Both will benefit performance, and either way you've gained."

I hadn't followed a training program this seriously since I'd played college soccer, but I realized that things had changed a bit in the past 20 years—namely, I'd gotten a life, as most of us do, and it had an annoying tendency to get in the way of my workouts. Travel, deadlines, friends, family, injury, illness, a weekend watching HBO's Sopranos-a-thon—each had threatened to derail my aerobic and strength improvements. The key, I learned, was to not let short-term downs derail long-term ups. "Consistency is everything," Henderson told me. "My athletes that put in the daily work over several years are the ones who see the best improvements."

I was particularly bummed when, a few days before the Mt. Taylor Quad, I caught a nasty bug that not only put me out of the race but floored me for days, like a right hook from Wladimir Klitschko. By the time I was back on my feet and training again, I'd lost three weeks and was starting to panic about the fast-approaching Grand Traverse. I'd participated in this event five years earlier, crossing some of the burliest terrain in the Rockies, and it had taken me a humbling 13 hours.

By the time my 33-year-old teammate and I lined up at the start (you're required to race in pairs), I wondered if my time in the pain cave—at BCSM and during my workouts since—had paid off. It's not like I expected to win the thing, since the field included some highly accomplished endurance athletes, but I was keen to see if I could improve on my previous time—when I had trained more but arguably not as well. In a way, this drilled to the core of what lifelong fitness meant to me: It was less about vanquishing my opponents (though that's always nice) and more an internal competition to see if I could identify a goal and then do what it took to get there. Me vs. me.

Starting at midnight, we skied through the night with 240 other racers, over 12,000-foot passes, in blowing snow, heads bowed into a fierce and frigid gale. Whenever I could manage, I'd sip an electrolyte beverage and gag down a packet of energy gel. I thought of what Henderson told me after I had bailed from the Mt. Taylor Quad: "The hallmark of a lifelong athlete is not his ability to focus but to refocus."

By the time we were swooping down Aspen Mountain toward the finish line, I was exhausted to the soles of my blistered feet, but I felt great, as fit and powerful as I'd ever been. Better yet, I knew we'd skied a smart, strong race. When we skidded to a stop in front of the cheering throng at the base, an official handed us a card with our time: 12:41—44th place and 20 minutes faster than our previous best, through some of the toughest conditions in the event's ten-year history. The winners, Stephen White and Mike Kloser, had finished in an astonishing 8:46. Their ages: 39 and 47, respectively.