The issue isn't calories so much. Even a very lean runner like Kipchoge has sufficient fat stores to walk this distance in a fasted state. The issue is glycogen

The issue isn't calories so much. Even a very lean runner like Kipchoge has sufficient fat stores to walk this distance in a fasted state. The issue is glycogen, which is necessary for maintaining aerobic performance. Glycogen is only stored in the muscles and liver and once you exhaust it, you switch to fat burning, which uses a different metabolic path and significantly slows you down. This is the infamous "hitting the wall." [4] In addition, you can only digest and metabolize a certain number of calories per time unit. This limits how many carbohydrates you can consume during the race. The idea behind carbo-loading [1] is to pack as much glycogen as possible into your muscles and liver before the race. You then top up those stores just before the race with more carbs, and then during the race you take on as many additional carbs as your body allows. The faster you run, the more quickly you use up glycogen, similar to driving a car more quickly. One thing that training does is to make you a more efficient/economic runner. [2] Your body can run faster using less fuel. I don't think we know exactly how all this works. Some of running economy comes from improved form, and indeed, Kipchoge has perfect running form. As well, your body's vascular system improves. You improve your mitochondrial volume. And so on and so forth. [3] But at the end of the day, you eventually run out of glycogen and slow down. You just hope you've picked a pace not too fast for your ability so that you cross the finish line before that happens. This is largely why ultra runners run at a significantly slower pace. Lastly, metabolic factors[5] are only one many things that ultimately limit marathon performance. 1. https://en.wikipedia.org/wiki/Carbohydrate_loading https://en.wikipedia.org/wiki/Carbohydrate_loading 2. https://runscribe.com/improving-run-efficiency-what-research-tells-us/ https://runscribe.com/improving-run-efficiency-what-research... 3. https://www.issaonline.com/blog/index.cfm/2018/mitochondrial-adaptations-to-aerobic-training https://www.issaonline.com/blog/index.cfm/2018/mitochondrial... 4. https://en.wikipedia.org/wiki/Hitting_the_wall https://en.wikipedia.org/wiki/Hitting_the_wall 5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2958805/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2958805/