I took part in a 100km run on the trails. It took me more than a day to finish the distance. I experienced something very interesting: the flow of time seemed to have stopped, and the usual measure of distance became irrelevant. The effort of running and hiking through the mountains warped my perception. This was useful because 100km feels like a very long distance until you break it down to this climb or the path you can see. Taken this way it becomes a hundred 1km runs - much more accessible. External measures of progress like pace also lose significance. You have to look at the effort you are making and keep it consistent. This becomes the measure of progress. Strangely, the going gets easier with more distance. Perhaps because I could no longer run and gained a more accurate understanding of how fast I could move. That made me relax mentally and physically.