What your failing to understand is the fine line difference between proper exercise and over exertion. Your heart needs to have mildly elevated demands place on it, so that it can "buff" up, just like your biceps
when you lift weights.
However, if you try to pull or lift more weight than your ready to handle, you'll get a muscle tear or strain the muscle. The same with the heart. If you exercise more than it can handle, you may get a heart attack
I would suggest that you speak with your doctor about what is the proper strategy for exercise, based on your individual health situation.