Its interesting how a report came out recently about how marathoners are more likely to get colon cancer at young ages. Apparently your stomach begins to literally bleed if you run to long in a single period.
Something about running that long fukks with our digestive system seriously. I sometimes wonder if on a physical level going that far at a single time would be the equivalent of running for your life, so your body reacts in desperate ways. Running marathons are not natural.