Apart from the elite corps, ultrarunners for the most part are not out there logging huge amounts of weekly miles. Most do not have the physiology or the time for that. Essentially this means that after a certain number of hours and miles, you are going to be feeling as bad as you can. Then the pain and discomfort will level off and sometimes you will even feel better for a while. This physiological certainty, along with many long training runs and tons of motivation, will usually do the trick.
Pacers are another huge aid to the runners.
Each entrant is allowed a pacer after 62 miles, someone who will accompany the runner the final 38 miles to the finish line. Psychologically, this can be a huge help, just having someone alongside to share the effort. How about directing the event. Norm Klein is at the helm of this race, organizing thousands of volunteers. There is almost an endless list of things that can go wrong, with 500 tired runners spread out over 100 miles of desolate mountain terrain.