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.