This is fascinating. I always thought of ketosis as a kind of breakdown status, when the body starts to "eat itself" after running out of fuel. And I have felt miserable the few times I hit the wall, wasted and lethargic, with the weird taste of metal in my mouth.
It never occurred to me that ketosis could be a long term option for functional nutrition, though I see it is an intended component of Atkins-style low-carb diets. The endurance running hypothesis is intuitively appealing, and Xavi makes a good point that persistence hunters would not likely have vast stores of glucose-rich foods like the high-octane gels and sugary sports drinks that fuel a modern marathoner.
50K in 7:45 is not an especially brisk pace, about 15 min/mile, but even walking that far after three days of fasting would have seemed unthinkable to me. And of course he does much of it barefoot.