The benefits of running are well publicized; aside from its stress- and depression-busting qualities (“runner’s high,” anyone?), the sport is also linked to improved immune function and reduced risk of diseases like diabetes and breast cancer. Stanford University even found that, after tracking 500 seniors for two decades, the running group on average suffered disabilities 16 years later than their non-running counterparts. But for every study that suggests the more running you do, the better, there seems to be another contending that more running is not healthy. Some people, due to a variety of factors from family history to poor biomechanics, may have a lower threshold for running — their knees may start to ache at 5 miles versus 15, for example. But genetics and poor form are only part of the equation, as several studies in recent years suggest that with running, you really can get too much of a good thing.