Paula Radcliffe and Haile Gebrselassie both hold world records for the fastest marathon times; both will be defending those records in Berlin’s annual marathon this weekend; and as ridiculous as it may sound, both are considered old.

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 cliche’ 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.