While everyone agrees that moderate levels of exercise is good for you, there's debate about the effects excessive endurance cardio. In fact, some research suggests that serious endurance athletes (we're talking about runners who log more than 30 miles each week) are no better off than their non-runner counterparts. The benefits of cardio seem to disappear when done in excess. But a small Circulation study contradicts these findings, saying that endurance training may actually deliver major heart benefits.
A team from the Saarland University in Germany examined the heart health of 33 competitive male "master endurance athletes"—including former Ironman competitors and Olympians—who each had an average of 29 years of serious endurance training. Using various cardiology and fitness tests as well as ECG imaging, they looked at the health of the athletes' hearts, with a particular interest in the right ventricle. They compared these findings with 33 non-athlete men of similar height and weight.
In doing so, they tested the previous theory that the ventricular function and heart size were negatively affected by long-term endurance training. And what they found was that contrary to previous research, elite-level endurance-trained men actually had larger and stronger hearts than the men who didn't train.
It's important to note, though, that this small study is hardly the final word on the topic. Make sure you find time for plenty of rest days and listen to your body when it's feeling overworked. (Speaking of rest days, here's when it's OK to work the same muscles back to back.)