Photo: lzf/Thinkstock

5 of 6

I'm all for cardio, but a review in Mayo Clinic Proceedings suggests that too much may be harmful. Researchers who looked at studies of marathon runners and other endurance athletes found that long, intensive workouts can sometimes cause structural changes in the heart, increasing the risk for conditions like arrhythmia. That's not to say you should put away your sneakers, but don't beat yourself up if you're not logging mega miles; just 75 minutes of running (or 150 minutes of brisk walking) per week is a fine benchmark to strive for, according to the CDC.