Marathon running is bad for you Marathon running is bad for you if you keep on doing it, doctors say. One is fine but after that it is best to restrict vigorous exercise to 50 minutes a day. High-intensity exercise continued over hours and repeated over years "stretches" the heart, disrupting muscle fibres and causing micro-tears. There are signs of damage after a marathon but these rapidly return to normal. If the damage is repeated, it can lead to scarring, US cardiologists say in the journal Heart.