So, I’ve read race reports from 100 milers who pointed to a realization in the race that they could walk it in and still finish under the time limit. For example, suppose you make it to 80 miles in 22 hours, and you are still capable of walking 3 mph. So simple math (the only kind you can do in your head at mile 80) tells you that even if you can no longer run, you could walk and still come in under 30 hours.
There are other thresholds along those same lines, say 90 miles in 25 hours and you can walk 2 mph. You get the idea.
But for me, the salient milestone is what I affectionately call “The Land of Single Digits Remaining,” and it works regardless of ultra race distance. Whenever I come to the point that there is less than 10 miles to go, I heave a mighty internal sigh of relief. I KNOW that I could practically crawl 10 miles if I had to. The remaining 10 miles tick off like clockwork and the finish line comes into view, and another memory has been made.