Metabolically, a 50-mile race differs significantly from a marathon. Elite marathoners can effectively rely on glycogen as their primary energy source, thanks to carbohydrate loading and strategic in-race fueling.
However, this approach is not feasible for a 50-mile race.
Success in such ultra-marathons hinges on a runner's ability to efficiently oxidize fat.
Being able to run a marathon in 2:10 doesn't necessarily translate into the ability to set world records in 50 or 100-mile races, as these longer distances demand a different metabolic proficiency.
This distinction is somewhat analogous to how the 400 meters and the 1500 meters differ metabolically.