The heart is a muscle, over stressing any muscle regardless of what it is can damage it, doesn't matter if it is your legs,arms, shoulders or your heart. It has nothing to do with age either. Tearing muscle down to let it rebuild stronger is the goal regardless if it is your heart or your legs. Over stressing a leg muscle can keep you off you bicycle for months, over stressing you heart can keep you off your bicycle for ever. I am not advising you to do anything, I can only tell you what I know from my years bodybuilding. I am no Dr but I do know the heart is the most important muscle in your body.