According to one research scientist too much excercise can lead to memory loss and premature senility (link via Barbelith Underground):

He added: “People who would rather laze in a hammock instead of running a marathon or who take a midday nap instead of playing squash have a better chance of living into old age.”

He said he was in favour of moderate exercise such as walking, but said excessive exertion was not recommended.

Middle-aged people should be wary of using up their energy on activity, he said.

Prof Axt said: “Research shows that people who run long distances into their 50s are using up energy they need for other purposes.”

He said they could suffer memory loss and risked premature senility.

Update: Axt has subsequently written a book called The Joy of Laziness. Judging by the Amazon reviews, it seems like his research is questionable.