The idea that more and more high intensity exercise, such as marathons, can only do you good, is a myth say the US cardiologists, and the evidence shows that it's likely to more harm than good to your heart.

"If you really want to do a marathon or full distance triathlon, etc, it may be best to do just one or a few and then proceed to safer and healthier exercise patterns," they warn.

"A routine of moderate physical activity will add life to your years as well as years to your life. In contrast, running too far, too fast, and for too many years may speed one's progress to towards the finishing line of life."