A study of forty endurance athletes found evidence of right ventricular damage after an endurance event.
"The study 'begs the hypothetical question whether repetitive longstanding bouts of arduous exercise result in the development of an acquired form of arrhythmogenic right ventricular cardiomyopathy.'" - Sanjay Sharma, MD, and Abbas Zaidi, MBBS, of St. George's University of London
Most of the runners had developed a sub-clinical fibrosis in the right ventricle of the heart. It is not easily seen, but it can cause fibrillation/arrhythmia, which may kill a runner.
It begins to answer the question of why some runners who appear healthy can suddenly die, when they showed no evidence of heart issues.
Here's what one former high-level marathon runner has had to say about the subject of chronic cardio.