While this may be possible, I would worry about the effects on your liver. Ketosis does put additional strain on the liver, and while it doesn't seem like the side effects of extended ketosis are dangerous, there simply aren't any studies on ultra-endurance exercise during ketosis. And while studies on extended exercise during ketosis would be a good thing, they should be performed by researchers and doctors who know what the risks are and can monitor them appropriately.

IMO this is dangerous. Just because our hunter-gatherer ancestors did it doesn't mean it's healthy for us. For all we know, this type of activity could wreck your liver and contribute to dying in your 30s (our ancestors did not have a long life expectancy). So even if it was a survival adaptation, it may have ensured the survival of the herd at the cost of the individual.