A friend of mine does keto and after running a marathon reported that they did not experience "the wall". Presumably this is because they were using fat to begin with, not after mile 15 or whatever.
For those interested, Jeff Volek's team published a study in 2016, "Metabolic characteristics of keto-adapted ultra-endurance runners" [1] that compared fax oxidation. One of the "fat adapted" participants Zach Kibbter, an ultra-marathon record holder, posted a personal writeup that included a table of his fat usage at various VO2 Maxes (a ridiculous 98% fat usage at 75% VO2max and 76% even at 84% of VO2max!)[2]
I don't know the numbers for the general population, but Fig2 [1] illustrates the difference between the high-carb and low-carb arms between the elite athletes, and Fig3 [2] shows the difference in absolute fax oxidation rates between the groups. If you're interested in finding out more, I'd also recommend doing a literature search for "maximal fat oxidation" (MFO, FATmax) - eg, here's a recent (2018) review w/ Fig1 that shows MFO peaking at 0.72g/min [3] (the FASTER trial cohorts avgd 0.67g/min for the HC arm, and 1.54g/min for the LC arm)
If you're interested in some of the other physiological impacts of keto adaptation, here's a presentation Volek gave a couple years ago that helps contextualize both the FASTER trial and a related TANK trial his team ran, as well as some other interesting related performance research: https://www.youtube.com/watch?v=BeS_dhM8dsY
It all has to do with intensity. We obviously can't tell from the information provided, but if they ran at a slow pace, that would also skew the fueling towards fat.
Exactly. While the marathon is an endurance event, the intensity when racing is often higher than 65% VO2 max - the "fat max" or point at which fat oxidation peaks. For example, for a 3 hour marathoner (or faster), the intensity would be around 74-84% VO2 max or 80-90% max heart rate. It has also been observed that going on keto reduces your running economy (so you end up running slower for the same effort).
As for "hitting the wall", this often happens around 20 miles as this is where your stored glycogen is depleted. To avoid this happening, you'd be better off to "train the gut" to handle consuming ~60g carbs per hour while running at your target pace/intensity.
Generally competitive endurance athletes try to shift their metabolism to match the length of their target event. For the longest ultra endurance events you have to be pretty well fat adapted because if you relied mostly on glycogen you wouldn't be able to take in enough carbohydrates during the event to avoid hitting the wall. But for shorter, high intensity events you're going to be burning mostly glycogen and barely touching your fat stores. It's a spectrum.
[1] https://www.sciencedirect.com/science/article/pii/S002604951...
[2] https://web.archive.org/web/20201111190720/https://zachbitte...