Another book recommendation:  Spark: The Revolutionary New Science of Exercise and the Brain by John Ratey, M.D.  Dr. Ratey is a clinical associate professor of psychiatry at Harvard Medical School.  In this book, he discusses the remarkable new research that is being done linking running to the effective treatment of depression, anxiety, ADHD, hormonal changes, and cognitive changes that develop with age.  He does a great job at making the research understandable to those not proficient in this arena and develops a compelling rationale for the positive effect of running on a wide range of mental disorders from childhood to aging adults.