"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 (Read the full article here.)
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.