Too much exercise can be bad for your heart. Very bad. In fact, too much exercise can cause permanent, irreversible changes that will damage your heart. You'll want to read the article, "Potential Adverse Cardiovascular Effects From Excessive Endurance Exercise."
Here are some of the reasons why you want to avoid excessive endurance exercise:
Elevated troponin levels
Elevated brain natriuretic peptide
Heart enlargement (hypertrophy)
Coronary artery enlargement
Cardiac scarring and fibrosis
Structural changes in the heart (cardiac remodeling)
What do we really know about all these things? Most of these changes don't develop overnight. Are there ways to prevent these cardiac changes if one is actively engaged in high-endurance athletics?
My personal goal is to avoid running over 20 miles per week. I also keep most of my training runs at 6 to 7 mph. This keeps my heart rate in the right "zone" so that I'm not engaging in highly strenuous exercise for extended periods of time.
I'll be running a half-marathon in a few weeks. 13.1 miles is a nice distance to keep me highly motivated. I don't know if I'll register to run a full marathon this fall because I seem to get more convinced by all the data that I may end up damaging my heart.