Is this speculation on your side, or do you have a source for this "biological reality"?
(1) The "endurance running hypothesis" suggests we may be exactly made for this. However, what's certainly unaccounted for is pushing limits by sidestepping physical and mental safeguards. Just fueling a run with highly available carbs is a situation "nature" couldn't possibly account for, since even most plants we eat today didn't even exist, much less highly caloric gels. Mindset is another dysregulated thing. Grave difference between hunting prey and records. You have to disregard feelings like exhaustion or lack of motivation to chase records. Not to mention, for most of history humans had way more leisure time to rest. Hunting was their job, not the hobby on top of demanding job. It was also a collective endeavor not a competitive one. So, the way it's done is certainly a perversion, but the ballpark of physical load may be within specification.
(2) I think the relevant time frame is too short to expect any pronounced evolutionary effect from civilizational slack. Endurance running may be a defining feature of our species and sedentism is a very, very recent development in evolutionary timescales. I believe, lactase persistence is a rare outlier of rapid adaptations within that time frame. And that's really a consequential but minuscule genetic change, compared to systemic degeneration of our evolutionary legacy.
On a side-note, I want to add, your understanding of "biological realities" regarding (2) has some history and is somewhat in line with prior thinking that ended in social darwinism and eugenics AFAIK.
> Might anyone know if indigenous groups such as the https://en.wikipedia.org/wiki/Rar%C3%A1muri - where extreme distance running seems part of their traditional culture - have been studied in this regard?
I have a hard time believing you made this connection, without knowing about the book Born to Run and the endurance running hypothesis, but anyway, I believe that's where the anthropological discourse is: https://en.wikipedia.org/wiki/Endurance_running_hypothesis