Can exercise be dangerous?
This is what you need to know.
When lifelong competitive endurance veteran athletes were compared with younger endurance athletes and control age-matched participants, they were found to have myocardial fibrosis which is abnormal changes to the heart muscle(Wilson M,et al. 2011).
This finding was significantly associated with the years spent training, the number of competitive marathons and ultra endurance marathons completed.
Keep in mind that this was hard endurance training and extreme endurance competition over a lifetime, it does not mean that exercise is not good, it just means that this type of activity does not seem to be the best.
Another study compared participants with coronary heart disease with different levels of physical activity(Mons U,et al. 2014). They found that participating in strenuous endurance exercises more than 5 hours per week increased cardiovascular mortality. Exercising less than twice per week also increased the risk.
What does this all mean? It means that it is important to be sure you get enough recovery time, and long hours of endurance training is not the best way of spending your time. You are more likely going to reap more benefits if you do high intensity, short interval training and include some resistance training the other days.
One of the important things to keep in mind is that recovery from the exercise is as important as the exercise itself. Without proper recovery it just adds stress.