More on "Running helps your knees?" and exercise

I received an email from Dan Peterson, reacting to my comment in my Aug. 27 post that I "feel strange if I have missed a day of going to the university gym to swim, run, or do weights." He had just done a story on a study by Robin Kanarek of Tufts University on the endorphin-fueled addictive qualities of running/exercise in rats. Here is Peterson's blog.