Ultra running (longer than a marathon) is not a test of endurance. Certainly, one must be in excellent physical condition to have a chance of finishing, but what makes or brakes you is your mind. When the halfway marker of a race is (1) after the part where you fingers went numb from the hurricane force winds, (2) farther along than any distance you've ever traveled on foot in a day and (3) after you long since passed the part when you were running on fumes.
That was the story on Saturday for me. Somewhere between Mt. Galehead and Galehead hut, going up and down over the various step cols and buttresses along Garfield ridge I was broken. Really pushing yourself, really pushing yourself, is finding out your real limits. Finding out when you can really run no more. I heard from other runners that some of their friends can jog out on the trail for days and days. After this weekend I suspect fewer than 1% have come anywhere near their real limit. At the very end of the race, I was able to jog across the final bridge to the parking lot where we started. If I could still jog, I didn't push myself completely. But at least I really know what that means now.