it's so hard to get good data for studies, because people's prep is so varied. A lot of people have marathons on their "bucket list" and yet they don't prepare properly for them. I know a group of people that worked for a year to be able to run a half-marathon, and then one of their friends decided to go with them and run it on no training. That person was at risk, but if he had he been a victim of SCD, he would have been counted as a marathoner. There is another thread in training and nutrition where someone mentions elevated body temperature as a risk of marathoning. It's not a risk of marathoning, it's a risk of failure to properly understand fairly simple things about your body. Of course, that's not uncommon, it happens to a few randonneurs every year. Randonneuring is low-intensity enough that people can ride without training enough. I know, I've done it. I don't see the potential ramifications of that as a risk of long distance, but rather a risk of improper training.
The arrhythmia issue is something I'm concerned about, but it doesn't seem particularly high-risk.