Ms. Radcliffe, at 37, and Mr. Gebrselassie, at 38, are over the hill by elite runner standards. Yet they are at the top of their game and among the favorites to win the standard distance 42.2-kilometer, or 26.1-mile, race on Sunday.

"Age for me is just a number," Mr. Gebrselassie said in a recent interview on YouTube with the organizers of the Berlin Marathon. "If you are old mentally, you are old physically. Automatically." He said that he feels 23 or 24.

Success in running is not just a mental feat, of course, it's physical, too. And the good news is that science backs up the cliché that age doesn't matter, or at least doesn't matter that much.

A few years ago researchers at the German Sports University Cologne took a close look at the finishing times of 400,000 marathon and half-marathon runners between the ages of 20 and 79. They found no relevant differences in the finishing times of people between the ages of 20 and 50. The times for runners between 50 and 69 slowed only by 2.6 to 4.4 percent per decade. "Older athletes are able to maintain a high degree of physiological plasticity late into life," the researchers wrote.

That might explain in part why the running world is growing, and growing older. The number of runners who finished marathons in the United States, where 7 of the world's 15 largest races took place last year, increased to 507,000 in 2010 from 25,000 in 1976, according to RunningUSA , an organization that promotes the industry.

In 1980, the median age for a marathon runner was 34 for men and 31 for women. By last year, the age had risen to 40 for men and 35 for women. People over 40 now comprise 46 percent of finishers, up from 26 percent in 1980.

For both Ms. Radcliffe and Mr. Gebrselassie, the race Sunday is a bit of a comeback. Ms. Radcliffe hasn't run a marathon since 2009, and has since suffered back pain and given birth to a son. She's using Berlin to qualify to represent Britain in the London Olympics next year. Mr. Gebrselassie pulled out of the New York Marathon last November after an injury. He then announced his retirement, only to take it back a few days later.

Whether you are an elite athlete or an amateur, the daily training to complete a marathon, triathlon or any long-distance event can be grueling and painful. There are debilitating shin splints, the risks of Achilles' heel, iliotibial band syndrome or plantar fasciitis. And there are fatalities. In August, a 64-year-old man died after a heart attack during the swimming leg of the New York City Triathlon.

So how can amateurs prevent injury and burnout to maximize athletic careers? There's no simple answer, but if you ask enough people the responses boil down to nutrition, moderation, discipline, setting goals, proper equipment and experience.

For Bob Rebello, a retired U.S. Marine living in San Diego, diet is key. He began running seriously in 1999 at age 63. Back then he suffered from high blood pressure, high cholesterol and he was 25 pounds, or 11 kilograms, overweight. He began to record everything he ate. "I was eating all the wrong food; potato chips, hamburgers, french fries," he said. He got rid of salt, processed foods, red meat and pizza. "I eat potatoes once in a while," he confessed, "but only small potatoes."

Mr. Rebello, now 75, was the oldest runner in the Antarctica Marathon in February, and has completed a marathon on each of the seven continents (raising over $81,000 for kidney cancer research in the process).

In Hals, in northern Denmark, Bjarne Jensen, 52, is a principal at a local school. So it may be no surprise that for him discipline has been the secret to prolonging his running years.

Mr. Jensen used to stop running during the brutal dark winters. But now he runs year round, five or six times a week, sometimes wearing cleats to get across the snow. His runs never exceed 20 kilometers, even in advance of a marathon when many experts advise running up to 36 kilometers in one go.

The constant shorter distances, he says, give him all he needs to do a marathon on a moment's notice. It seems to work. He has run 127 marathons so far, 21 last year alone. His goal is to run 25 marathons this year.

Arguably the most important tool to keep you running over the years is appropriate shoes. Koen Wilssens, 30, operates a chain of running shops called Runners Service Lab in Belgium. His selling point is an indoor track of some 40 meters that scans the foot as it hits the surface to determine the type of shoe one should wear. (Ms. Radcliffe appeared for the opening of the newest location, just outside Antwerp, because that's where she buys inserts for her Nikes.) http://www.rslab.be/

Mr. Wilssens estimates that 50 to 60 percent of people wear the wrong type of running shoe. Not everyone has access to a shop like his, but many metropolitan areas have shops that offer video analysis. The right shoes won't guarantee you first place, but the wrong shoes guarantee you a short-lived running career.

Thom Gilligan's livelihood is running. A former competitive racer, Mr. Gilligan, 62, founded Marathon Tours & Travel in 1979 in Boston, which takes groups to races as far-flung as Kenya and Antarctica. For him, the hardest part is "keeping the passion going." His solution is to set achievable goals.

And sometimes, success is simply a result of years of experience, as it is for Heiko Schaefer. Mr. Schaefer, 72, is busy trying to qualify for the Six Foot Track Marathon , a 45-kilometer race in the Blue Mountains west of Sydney.

Mr. Schaefer began running for fun 21 years ago with his son. In 2000 he joined a running club, the Sydney Striders, and a few days afterward ran in the Host City Marathon along the course designed for the Olympics that were held in the city later that year. "That was the day I caught the bug," he recalls. He drew on the knowledge of his teammates and read everything he could about running.

In April, he won the over-70 age group at the Canberra Half Marathon with a time of 1:47:32. As his name was called and he approached the podium to collect his prize, he was expecting a trophy or a medal. Instead, he received Paula Radcliffe's book "How to Run." "There was nothing in there that I didn't already know," he joked.

All this advice is worth heeding. I ran my first marathon in Rome in 2004, when I was 34. The race started at the Colosseum, went past the Vatican as the pope was preparing Mass and ran through Piazza Navona. As spectacular as those sights were, one of my most vivid memories was of the older runners, many in their 50s and 60s and 70s, passing me as I began to struggle at Kilometer 30.

This weekend, at the age of 41, I will be joining Ms. Radcliffe and Mr. Gebrselassie in the Berlin race, though thousands, if not tens of thousands, will cross the finish line between them and me.

I am fitter and faster than I was in Rome, which is a good feeling. But much of my satisfaction stems merely from being able to keep running, because a lot can go wrong along the way.