Incidentally, the referenced book by David Linden (a professor of neuroscience at the Johns Hopkins University School of Medicine), "The Compass of Pleasure: How Our Brains Make Fatty Foods, Orgasm, Exercise, Marijuana, Generosity, Vodka, Learning and Gambling Feel So Good" is excellent.
From The New York Times:
WELL: The Evolution of the Runner's High
People and dogs experience runner's high. Ferrets don't. New findings may help explain why aerobic exercise appears to be part of our evolutionary history.
http://nyti.ms/I9HHCR