Radcliffe, is 37, and Gebrselassie, 38, but whenever they line up at the start of a marathon, they are among the favorites to win the standard distance 42.2-kilometer, or 26.1-mile, race.

“Age for me is just a number,” Gebrselassie said in a recent interview on YouTube. “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.

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 US 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.”

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.
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. ( Radcliffe appeared for the opening of the newest location, just outside Antwerp, because that’s where she buys inserts for her Nikes.).

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, 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.

Gilligan’s personal goal is the Bermuda Triangle Challenge, a January event that he’s entered 33 of the past 34 years. It consists of a mile race on Friday, a 10-kilometer run on Saturday, and a half or full marathon on Sunday. “Any person who has been fit all their life I think realizes the intangible benefits of having a healthy lifestyle,” he says. “It’s addictive.”

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

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. 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.

Last weekend, at the age of 41, I joined Radcliffe and 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 now, 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.

Where to run in seven continents

* Africa: Old Mutual Two Oceans Marathon, South Africa. Bills itself as the world’s most beautiful marathon. Ultramarathon with 56 kilometres, or about 35 miles, of spectacular ocean views around Cape Town. Held on April 7.
* Antarctica: Antarctica Marathon. Sold out years in advance, the race offers runners the chance to face off with ice, mud and penguins. March 9.
* Asia: Great Wall Marathon, China. Run the 5164 stair steps and kilometres of dusty roads into rural villages. May 19.
* Australia: The Solar Eclipse Marathon, Port Douglas, north of Cairns. The race begins along Four Mile Beach at around 6.30 am when the sun reemerges from behind the moon. November 14, 2012.
* Europe: TCS Amsterdam Marathon, Netherlands. The super-flat course is divided between stretches in town and along the serene Amstel River. Oct 16.
* North America: Big Sur International Marathon, California. Mountains on one side and the Pacific on the other make this a perennial favourite. April 29.
* South America: Inca Trail to Machu Picchu Marathon. A 27.5-mile (approx 44.26 kms) route in the Andes. Various dates.