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Running too far too fast, especially in middle age, may take years off your life instead of improving your health. A new review of research shows that while regular runners lower their risk of early death by nearly 20 percent compared with non-runners, running more than 20 miles a week can actually be harmful and lead to cardiac damage. Running too fast—at a pace faster than eight minutes a mile—also seems to stress the heart. “After age 50, pushing too hard is probably not good for one’s heart or longevity,” James O’Keefe, a sports cardiologist at Saint Luke’s Mid-America Heart Institute in Kansas City, Mo., tells The Wall Street Journal. A growing body of research shows that extreme exercise can harden the coronary artery, a condition typically seen in people who are completely sedentary. Endurance athletes—such as marathoners and triathletes—also appear to be at higher risk of atrial fibrillation, a heart arrhythmia responsible for one in three strokes.