Camille, really appreciate your posts and of course greatly respect what you have done at Comrades and the 100-mile etc.
However, I don't understand your point on stored glycogen and these distances of depletion in an ultra? Many people (regardless of pace/intensity) will "hit the wall" and run out of stored glycogen at mile 20-22 of a marathon. It is a simple work output. We're always riding the line thin line of bonking in the latter stages of an ultra or marathon if we are pushing hard.
Now a well trained (high mileage runner and long run adapted to burn fat as fuel more efficiently) athlete who paces themselves well in a marathon/ultra wants to just "brush up against the wall" ideally. Of course this only happens when one executes a very good race. Despite running at sub maximal paces and trying to utilize a high % of body fat to burn as fuel they are generally also taking in carbs during the race so that they do not go hypoglycemic. Didn't you even have some beer in your 100-mile record?!
In my mind whether one "hits the wall" at mile 16 or mile 48 depends on: how well trained they are, how well they are managing their intensity/pace, how many calories/carbs they are ingesting during the event, and how efficient their running economy is at different points in the race. Of course with sheer muscle failure or something like dehydration this can alter form to become very inefficent and cause debilitating cramps/stomach issues.
I think Comrades are Two Oceans are more likely to throw some "Fast marathoners" for a loop because they have significant hills. Some runners are definitely not as good at climbing. We use Comrades because it is one of the fewer ultras where a decent number of sub 2:20 marathoners (and sub 2:40 for women) have raced....and because it is not on a trail. I think 50-miles/100km on a flat track/road would have a better conversion for the super fast marathoners.
And to the Forest Gump poster: Dude you are obsessed with these "blisters" as a limiting factor?! Get a well fitted shoe, some of my Drymax "Sage Runner" socks and lube up with some Squirrels Nut Butter and you don't have to worry about blisters anymore!