Here goes my theory, when the body is running low on glycogen (simple carbohydrates, easy to use for energy and movement, sugar) perhaps 200-300 calories of glycogen left, your body goes into conservation mode, and most people slow down. I ended my two previous marathons while still in this phase. I think that at this phase the body starts looking for easily available fuel, like more fats and proteins to use. It is still burning mostly glycogen, but instead of 75% at ideal marathon pace, maybe 60%. Now, myself going out at 5:31 pace the first 10 miles I was probably burning 80-85+% glycogen. The goal would be to enter into the low glycogen phase in the last mile or few miles of the marathon. This is the phase I would say you can gut it out, and hold a respectable pace. I imagine that Matt Tegenkamp and Moses Mosop were in this phase Sunday. The person is running slower, maybe 15-30 seconds slower than goal or average pace per mile.
However, eventually you run out of those remaining sugar calories, and the body has to use whatever remains, mostly fats and proteins. We have plenty of fat, I ran and walked 93 very hilly miles under 32 hours way back in 2010, using a lot of fat. The problem is, at marathon pace (5:30-6:30 pace at the time) the body is using a lot more energy per minute than trail running and walking (12-15 minute pace). That means instead of using fat, and metabolizing more sugars from eating, the body zeros in on protein. While being vegan for 73 days I did get enough protein, I certainly was not getting much extra. That’s fine, probably even healthy to get 75 g of protein instead of 250 g of protein daily, unless you plan on needing protein to run the last four miles in a marathon. While I was running the last four miles I felt like a lead balloon. My mind was not functioning as well as normal, I had hardly any energy. At the end of most races I feel like a lead gorilla, strong but my muscles are tired, this time I felt weak. I tried to pick up the pace, but after a step or two there was nothing there. My right leg is enormously sore, even painful, more than four days later. It started hurting the last three miles, but nothing terrible at the time. However, as my legs metabolized protein, and I did not get a whole lot of protein in the hours after the race (probably only 30 -35 g in the seven hours after the race, fortunately all in the first 90 minutes), my recovery will probably be enormously difficult and take a long time.
I cried in the last three miles and at the finish. However after getting a few tears out I am all smiles. Now I know. I have been to the other side. Hopefully I never go there again.