According to a press release by the American College of Cardiology, "new data show regular marathon runners have increased stiffness of the large arteries, suggesting that some types of higher intensity exercise may negatively impact heart health."
"Our data suggest that exercise may have an inverted U-shape relation with arterial stiffness. In other words, when you do not exercise you have higher risk of cardiovascular events, but the same also happens when you exercise too much, said Despina Kardara, M.D., Athens Medical School, Hippokration Hospital, Athens, Greece, and lead investigator of the study. Regular long-term endurance training is generally beneficial for heart health, but it seems that the cardiovascular system is like a sports car engine. If you do not use it, it will decay, but if you run it too fast for too long, you might burn it out."
"Researches say there may be several explanations for arterial stiffening in Marathon runners. One plausible theory is that extreme exercise may place repeated and excessive stress on the artery wall leading to its fatigue."
"Ebndurance athletes should be cautious about the amount and volume of their training programs, trying not to wear themselves out, and always work in close collaboration with their physicians, especially before participating in an intense endeavor like marathon running."