For Meb Keflezighi, above, and Bernard Lagat—the two ageless wonders of the U.S. men’s track team—the key to longevity seems to involve a relaxed attitude and a lot of rest. PHOTOGRAPH BY TIMOTHY A. CLARY / AFP / GETTY

In 2001, a professional runner named Gregory Jimmerson headed up to Mammoth Lakes, California, to spend a couple of weeks running with his old friend and rival Meb Keflezighi. The two were born three months apart, and they’d been roughly as good when younger. Both had come in second at the national high-school cross-country championships as seniors—Jimmerson in 1992 and Meb in 1993. As a senior at Stanford, in 1996, Jimmerson had been the fastest American at the N.C.A.A. cross-country championships. Meb, who attended U.C.L.A, finished a few places behind. The next year, Meb, who is universally known in the running world by his first name, won the race.

When he went to Mammoth Lakes, Jimmerson was still training intensively and had Olympic aspirations. But he had studied engineering in college and had begun working forty-hour weeks at a company that produced lasers. Meb focussed only on running. They worked out together, but Meb’s routine after he took off his shoes startled Jimmerson. “He would finish the workout and immediately get a protein shake, and then he’d go sit in an ice bath or the creek,” Jimmerson told me. “Then he’d get massaged and stretched and do core workouts. Then he’d take a nap and do it again.” Meb even had a tiny pool where he could run in place, Jimmerson noted, allowing him to get in additional workouts without pounding his legs. The sport itself doesn’t take that much time: Meb was running only about twelve hours a week. But he had turned his preparation into a full-time job. Jimmerson realized he didn’t have that kind of commitment. “It was an all-consuming life style with a singular focus,” he said. “That’s where his strength was.”

After those two weeks, their careers diverged. Jimmerson competed in the Olympic trials in 2004, in both the marathon and the ten thousand metres, but he was far behind Meb in both. He didn’t make the team, and he stopped running professionally soon after. He has moved on to a successful and fulfilling career in mechanical engineering. Meb, meanwhile, won a silver medal in the marathon at the Athens Games, and in 2009 he won the New York Marathon. In 2013, he was near the finish line of the Boston Marathon when the bombs went off. The sounds, he said, reminded him of his childhood, in Eritrea, which he had fled with his family, at the age of twelve. The next year, he went back to the city and became the first American man to win that race in thirty-one years. Now, at age forty-one, he’ll be representing the U.S. Olympic team in the marathon, this Sunday, in Rio. I spoke with him on the phone recently and asked why he’s still running. “I have God-given talent,” he said. “And I love to do it. I love exercising. I love the natural high.”

Meb is probably the most accomplished American marathoner ever. But his longevity may be even more impressive. Bodies aren’t supposed to run that fast for that long. For men and women, at age forty, the lower back aches; knees don’t quite bend the way they’re supposed to; a hamstring just stays sore, even after ibuprofen and ice. The decline seems to start at about twenty-six, or perhaps it’s twenty-eight, or maybe even twenty-eight and four months. This is similar to other sports. Basketball players apparently peak at twenty-seven. Baseball players are about the same, though some skills, such as baserunning, decline more quickly than others, such as walking. Tennis players start to falter at about twenty-four. There are exceptions, of course. David Ortiz can barely walk, but he’s still hitting .310 at age forty. Kristin Armstrong just won a gold in cycling, the day before turning forty-three. Still, Father Time remains undefeated in every sport.

So how did Meb, a man born the same year as Drew Barrymore, come to represent the United States in the most gruelling track event at the Olympics? For the answer, it helps to go back to Mammoth Lakes. First, there’s the obvious lesson: the man living in isolation at altitude with the stretching ropes and the singular focus ended up with the Tuck Everlasting career.

But there’s something subtler going on as well. What were the things that Jimmerson wasn’t doing that Meb was? Both were running themselves to exhaustion. But Meb was icing, stretching, working on his form, and taking naps. He was not merely focussed: he was also relaxing. He wasn’t obliterating a punching bag or poking voodoo dolls of his competitors. He wasn’t trying to run a hundred and thirty miles in a week, and then a hundred and forty. He did everything steadily and committed many hours every week to what he calls “prehab,” or injury prevention. Meb recently published a book that features a section on how exactly a runner should nap. He suggests, too, that you should drink a glass of warm milk about an hour before going to bed.

According to Michael Joyner, a physician at the Mayo Clinic who has written extensively about running and aging, there aren’t many reasons we have to get slower as the years go by. We suffer slight declines in what’s called our VO2 max, a measure of the body’s ability to use oxygen. But for someone who trains hard—and particularly, on occasion, in short bursts, called “intervals”—those declines are rather small. The other factors that determine speed—our form and efficiency, the amount of lactate that accumulates in the blood given a certain level of effort—should stay stable. “There are only two ‘secrets,’ ” Joyner told me, when I asked how a runner can continue to succeed until age forty and beyond. “Keep your VO2 max up by doing intervals, and don’t get injured.”

Meb isn’t the only American athlete in Rio to follow this advice. There’s another man, Bernard Lagat, who is also forty-one, and who finished third at the N.C.A.A. cross-country championships the year that Meb won. At the 2000 Olympics, Lagat won a bronze medal. In 2004, just like Meb, he won a silver. His main rival in those years, who is the same age, has been retired for more than a decade. This past month, Lagat obliterated his opponents in the five-thousand-metre run at the U.S. Olympic trials, running his last lap in just under fifty-three seconds. The second-oldest man in that race was thirty-one. Lagat runs his first race in Rio on Wednesday morning.

Lagat’s training routine, like Meb’s, seems to involve a relaxed attitude and a lot of rest. Lagat skips a day of running every week, and he takes a month off every fall. At most, he runs about sixty-five miles a week, which is roughly half as many as his most obsessive rivals. He’s not lazy—he does everything hard. He also never seems to get hurt.

The two ageless wonders of the U.S. men’s track team have other advantages, too. I spoke with Vin Lananna, the head coach of the U.S. Olympic team, and he noted that each has stayed with a single coach for the past twenty years. “When you are working with the same person, you get to speaking the same language. You say, ‘I have this issue with my Achilles,’ and they know what that issue with the Achilles is.” It may not be a coincidence that the two great American runners who recently burned out—Alan Webb and Ryan Hall—switched coaches. As he got older, Webb seemed to swap them out like pairs of shoes. And, late in his career, Hall, the fastest American marathoner ever, declared that only God would coach him. He retired this past year, at the age of thirty-three. Galen Rupp, who beat Meb at the U.S. Olympic marathon trials, has worked with the same man, Alberto Salazar, since ninth grade.

Neither Lagat nor Meb is likely to win gold. Lagat is facing maybe the greatest 5K runner in history, Mo Farah. The fastest marathoner in the field this year, Eliud Kipchoge, has run a time that would put him across the finish line when Meb, running his fastest time, would still have a mile to go. But both Americans have another advantage that comes from being in the sport so long: nothing will surprise them. Lagat timed his final sprint perfectly in the Olympic trials; Meb defied everyone’s expectations when he surged out ahead of much-faster runners early in the race and held on to win the Boston Marathon, at the age of thirty-eight.

When I asked Meb what it’s like to run at this age, he said that the secret is just to keep going. “We all have good days and bad days, and as we get older it takes a while to get going,” he said. “If you’re forty, or fifty, or sixty-five, the hardest thing is just to get out the door, and then to do it again.”