Basically, somewhere around 20 miles (for most of us mortals) you run out of readily available fuel (glycogen stores) and your body resorts to burning primarily fat, which is a much less efficient process than burning glycogen. This is colloquially known as "the wall". At that point you may encounter relatively high levels of both physical and mental fatigue. Your feet may begin to feel like concrete blocks and your legs like immovable pillars. That's hyperbole of course, but at that point you may find it apt. I've never experienced the mental side of it other than just wondering if it's all worth it and should I continue. (There is something in me that always insists that I started, so I WILL finish.) But I've read of other issues such as losing concentration and a kind of mind-body disconnect, and even to the extent of minor hallucinations. Again, maybe hyperbole, since I've never experienced these things.
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