All things in moderation, even exercise.  A new study from the University of Illinois at Chicago and Kaiser Permanente found that people who exercise more than 7 hours a week are twice as likely to suffer heart disease.  White men in that group were 86% more likely to experience arterial buildup by middle age.  Researchers tracked more than 3,000 people over 25 years.  

The team also notes that they aren't telling anyone to stop exercising.