I have some mates that compete in international ultra marathons that involve running between 50 & 90km per day for 5-8 days at a time. It's an amazing level of fitness and conditioning to be able to run that many KM day after day. I would love to have that level of aerobic conditioning.
I am in complete awe of their capabilities, but I doubt I'd have the motivation to run for such long periods. Then again, these two friends are a father and son team running to raise money to find a cure for diabetes and celiac disease that the younger son/brother suffers from quite severely.