The “average” runner on a “typical” course burns through about 100 calories / mile. So, in a marathon, you burn through about 2,600 calories. I was thinking of all the delicious ways to put those calories back on, when suddenly some things started to make sense. I put the wall, which usually hits at about mile 20, with the 2,000 calories you’ve burned so far, and put that with 2,000 calories the average person burns through on a typical day. So, in some sense a 20 mile run is within the bounds of reasonableness for your body.
I found this fascinating! No wonder a lot of marathon plans have the longest run being 20 miles. And no wonder your body does “weird, wild” stuff before you get to 26.2 miles. Because burning 2,600 calories is in no way typical. No matter what we are capable of doing, we (the vast majority of us) just don’t practice running 26.2 miles often enough to get used to it.
26.2 miles is well outside of typical. 26.2 is even bigger than 24, the number of hours in a day.