Yes, and the book "The Haywire Heart" documents the various research studies. It seems that certain high mileage individuals develop heart problems leading to AF, however there is nothing that points to a specific type of individual (no history of heart problems, no family history, all were healthy, etc.). The authors take the side that high mileage carries increased risk, so why do so much (even though they are runners). I didn't get an idea of the percentage of total high mileage runners and those of that group that developed heart problems. My take (opinion/guess) is that high mileage over a significant period of time does cause heart damage in certain individuals, but it may only affect about 5% of that high mileage population. Symptoms of heart issues were noted by those affected, i.e. palpitations, racing heart, skipped beats, dizziness, etc., that seemed to present early warning indications (but being healthy, high mileage runners, of course many chose to ignore them).