There is no question that regular aerobic exercise is good for you.  The question is, if regular exercise is good, is more exercise even better for you?  My guess is that the answer is "no."  That doesn't mean that those who exercise a lot are worse off than couch potatoes but they probably have shorter lives than those who exercise modestly.  

Yes! Now I can run again.

 

But I think the question has always been between moderate exercise and "extreme" exercise. Some articles (so sorry, so not going to google right now) I've read make it seem like it doesn't take much to be considered extreme, though I'm not sure I'd ever be overly considered about it in my case.

I think extreme is anything beyond C25K.

This has probably been linked already, but I was thinking about it on my run today (not about this study, but the previous one that was in the news):

Alex Hutchison's take on the "too much running is bad for you" study

The key excerpt: 

"In a study involving 52,600 people followed for three decades, the runners in the group had a 19% lower death rate than nonrunners, according to the Heart editorial. But among the running cohort, those who ran a lot—more than 20 to 25 miles a week—lost that mortality advantage.

But here, from the actual abstract, is the part they never mention:

Cox regression was used to quantify the association between running and mortality after adjusting for baseline age, sex, examination year, body mass index, current smoking, heavy alcohol drinking, hypertension, hypercholesterolemia, parental CVD, and levels of other physical activities.

What this means is that they used statistical methods to effectively "equalize" everyone's weight, blood pressure, cholesterol, and so on. But this is absurd when you think about it. Why do we think running is good for health? In part because it plays a role in reducing weight, blood pressure, cholesterol, and so on (for more details on how this distorts the results, including evidence from other studies on how these statistical tricks hide real health benefits from much higher amounts of running, see my earlier blog entry). They're effectively saying, "If we ignore the known health benefits of greater amounts of aerobic exercise, then greater amounts of aerobic exercise don't have any health benefits.""

It appears that the researchers are trying to isolate running as an independent factor- which is why they are 'equalizing' everyone's weight, blood pressure, cholesterol, and so on.  On one hand, I think it is important to try to separate variables to identify which factors actually cause particular health effects and why.  On the other, running is not independent.  It affects other things.  You can't compare a person who has OK but not ideal blood pressure, cholesterol, etc. and runs 25 mpw to a similar person who doesn't run-- the non-runner might have an ideal bill of health if they did run, and the runner might otherwise have terrible blood pressure, cholesterol, etc. but they keep their health factors in check by running.