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Coaches recommend slowly increasing mileage by increments during training to acclimate yourself to longer distances. Training programs are widely available online for novices and veteran runners.

Two experts who reviewed the research had opposite reactions to the finding.

"This study helps to further elucidate the potential cardiac dangers caused by excessive endurance exercise," said Dr. James O'Keefe, a sports cardiologist at the Mid-America Heart Institute, in Kansas City, Mo.

He supported the idea of vigorous exercise but disagreed that more is better. "Shoot for about 30 to 60 minutes a day of moderate or vigorous exercise, or 150 minutes accumulated over each week," he said.

O'Keefe advises those who want to run a marathon ''train up for it adequately and then do it. But then cross it off your bucket list, and settle into a regular routine of moderate exercise."

For the highly competitive, he suggests running 5Ks, 10Ks or an occasional half-marathon.

"I feel like that is about the upper range of what is not damaging to the heart," O'Keefe said of the 13.1-mile distance. "Even so, I wouldn't recommend half marathons on a regular basis, especially over age 45." He advised keeping half marathons to one or two a year, at most.

Another expert took a different view.

"I don't think it is reason for concern," said Dr. James Eichelberger, an associate professor of medicine in the division of cardiology at the University of Rochester School of Medicine and Dentistry, in New York. Training, he agreed, is crucial to minimize cardiac problems.

"All these findings [in the study] were mild and transient," Eichelberger said. "There is little risk. If you want to avoid that, exercising less vigorously for a less amount of time makes some sense."

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