Too Much Of Something
Too much of anything, even a good thing, can result in more problems. For example some experts say that too much exercise can cause oxidative stress in the body. Certainly most feel that some exercise can be important, but too much of it can be damaging to the body. Also for those that have not exercised in some time, too much exercise can cause their bodies to overload.
The key may be more in doing something consistently than just driving full force into something only to stop for awhile and then start again later on at the same pace. By slowly getting yourself started and working your way up to a comfortable workout level, you could end up doing much better in the long run and may be more apt to stick to a program rather that quit it right after you start.
So check with your doctor or a fitness expert and get their opinions. You have to remember also that each of us is different in many ways. Sometimes some trial and error may just be what the doctor orders until you have figured out what will work best for you and your body type.