A lot of ultrarunners are into the paleo diet, so I think you must just be reading a particular subset of the paleo people. (Maybe the Crossfit people? They are often paleo, too.) OTOH I think paleo is...not really very paleo. That is, when I've looked at what 'paleo' diets consist of, often they have, oh, almond flour cookies so as to avoid the evil of wheat - yet almonds are only very recently domesticated, and cereal grains have been cultivated for thousands of years, long enough that I imagine (much of) humanity is used to them.

I tend to go with the Michael Pollan 'diet' - eat food, not too much, mostly plants. (Well, I eat a lot of meat, too. But probably mostly plants.) I also apply a sort of inverse version to my exercise: run miles, a lot of them, mostly slowly. I run some very fast hard intervals (actually, mostly hill sprints and hill work as my 'hard') but mostly I just run at an enjoyable pace.