Is too much exercise harmful?

A few days back, during a breakfast meet with a friend, his pretty wife asked me whether I valued the need to exercise or not. She also told me about some long distance runners with poor life span, and asked me why such fitness freaks died so young. And I started to wonder. Can too much exercise be bad for you the same way medical science accepts that though a small amount of alcohol is associated with longevity but too much alcohol with early death?
I note that there are studies that tell us that a moderate running regime of two to three hours a week appears best for longevity. Matsumara and his colleagues conducted a large study on it involving 3,800 runners, both men and women above 46 years, who were reportedly running 20 miles a week.
Naturally when one does a study like this, one needs to take into account dangerous pain killers, high blood pressure, high cholesterol and diabetes, family history of heart disease and smoking as these factors shorten your life span irrespective of whether you are a fitness junkie or not. Taking all into account, a shortened life span for excessive runners was then documented.
A real reason as to why it is just conjectural; and reasons like excessive wear and tear, remodeling of the heart in such a way as to undermine some of the benefits of moderate exercise has been suggested. To be fair, these studies also suggest that cardiac markers of heart damage, similar to that you get after a heart attack are positive, presumably due to tearing of heart muscle.
Similar studies in mice also suggest that an iron mouse, made to run long distances for four months, also suffered such heart damage, but improved after it stopped running. MRIs of the heart done in persistent long distance runners also show signs of scarring.
Another study looking at 52,600 people followed up to three decades, including 14,000 runners and 42,000 non runners, tells us that moderate runners had a 19% lower risk of death.
However, those that ran 20 to 25 miles a week had the same death risk rate as a couch potato. A survey done in Taiwan on 416,175 adults for more than eight years showed those who exercised approximately 92 minutes a week had a 14% reduction in all cause mortality. For each 15 minutes, beyond a baseline of 15 minutes a day, a further reduction of 4% could be found. What was surprising was the study showed a longevity benefit at even 13 minutes a day.
A Stanford and Harvard study done in 1986 also addresses this issue. Around 16,000 Harvard alumni were studied and it was determined that in individuals exercising from 500 to 3,500 Kcal per week, there was a steady decline in death rate as exercise increased. There was, however, a moderate increase in death rate in those who exercised beyond 3,500 Kcal per week.
On the other hand, when researchers looked at cyclists who were part of the Tour de France between 1930 and 1964, they found in them a 17% increase in longevity as compared to the general population. It gave rise an important question: Do all sport provide equal benefit on longevity, or do some, beyond a point, produce detrimental results?
Another question that needs to be addressed: what is more important duration or intensity of exercise important for prolonged longevity?
This seems to be a not very well studied area, but if we believe a Danish study, workout intensity may be more important than duration when it comes to longevity.
The views on this are still conflicting, but my view would be that both are important. Other authorities seem to think that there is diminished return after 45 minutes of exercise and Swedish researchers have shown an increase in irregular heart beats in 30% skiers, who were in the fastest times in a 56 mile marathon.
So we come back to the same questions again.Is too much exercise bad for your health and does one have to hold on to the old adage that everything is good in moderation? So until we know for certain, do run, but not so excessively as to cross your life line.