The rare trends in data that suggest otherwise look like artifacts. The trends have not been confirmed, and in most studies did not appear at all. However, if you have symptoms, such as high blood pressure, a family history of heart disease, then you would be wise to avoid high intensity efforts at first, and have your doctor monitor your health and exercise program as you progress. If you are in good health and do not have any risk factors, there does not seem to be any compelling reason why you should not exert yourself as much as you enjoy. But we are all mortal, and there is no such thing as guaranteed protection from disease.
Sudden deaths do occur even in people in good shape.
For most of us, the psychological benefits and self-esteem earned by an occasional maximal effort far outweigh the tiny risk of serious harm. If you are not happy with this thought, then perhaps you should choose to stay in the moderate zone. Copyright, American Running Association.