A team of researchers and runners from the Heart and Stroke Foundation used MRI data to see how marathoners' hearts were effected by miles and miles of running. And what did they find? The magnitude of abnormal heart segments was more widespread and significant in a group of less fit runners. During the marathon, these less fit runners had signs that the heart might be at greater risk of damage than that of runners who had better training or at least had better exercise capacity (VO2 max).
The bottom line? Without proper training, running 26.2 miles can damage your heart for a limited amount of time (researchers put recovery at three months). However, if you've gotten the OK from your doctor, train regularly, have been running for years, hydrate well and eat well, you're probably in the clear.