I had a heart attack while riding in November 2000 at 46 YO. It probably only happened as I was doing long max effort racing my daughter up a steep hill. During the cardiac cath they had trouble finding the clogged artery as the doctor said my heart looked like a 25 YO's. If was a small, out of the way artery that clogged for some unknown reason and they couldn't even get to it to do a stent. So I live with it.

My cardiologist says that my excellent collateral circulation from exercising probably covered up the problem. She also said that aerobic activity, by itself, probably doesn't prevent heart attacks, but it definitely makes them much more survivable.