Perhaps I can claim to be the house specialist in ultras, then. I don't have any special knowledge about health and safety, but my pet theory is that overtraining is the biggest risk factor for runners at any distance. Perhaps logging fifty miles in a day causes no more cumulative damage than covering the same distance over a week, running every day or two. (At least not for back-of-the-pack finishers like me, who prioritize having fun and not getting hurt.) But the more frequent outings must interfere with recovery. It is rare that I run more than once a week, and I am even lazier in the summer and winter. I suspect that the shorter races, usually on carefully-measured pavement courses, tend to inspire more competition and focus on finish times, while the longer events are typically off-road, with irregular distances, and are perceived more as fun challenges. Emil Zatopek, the Czech Locomotive, seems to have performed better in competitions after illness or injury forced him to back down from his famously aggressive training regimen.