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.