A cardiologist who studied the results believes 2.5 hours of running over seven days may be the magic number. The suggestion is to spread that over several days and to keep the intensity to a moderate pace.
Dr. Martin Matsumura from the Cardiovascular Research Institute at the Lehigh Valley Health Network led the study.
He admits the biggest mystery is why too much running is bad for you.
However, doctor and running coach Jason Karp thinks he may have an idea. Karp told NBC San Diego that exercise as extreme as marathons can "cause some scarring of the heart."
The Pioneer Press reports other recent research results that you may find surprising: Male marathon runners actually had more plaque in their coronary arteries than non-runners.
One of the scientists involved in that study put it this way: “It is plausible, not proof by any stretch, that metabolic changes when running could be moderately toxic to arteries."