Good ultra runners are uniquely strong in their belief that they can finish any run. Nobody doubts that they can finish a 10K, few ever drop from a marathon. But when you are standing at the starting line, and consider that the finish line is 100 miles away, it takes mental strength to believe that you can cover that distance. Any runner who believes that he or she can't cover the distance will find it a self-fulfilling prophecy. It is common to feel very tired after running 40 to 70 miles.
Some runners give up, while others rise above fatigue and finish.
Finishing a run when you thought you were dead meat is one of the great joys of ultra running.