Karnazes is of course an ultra-marathon legend. I've been to one of his seminars. After the lecture I ask him his "secret"- he motioned for me to step closer and then looked around, as if to make sure nobody else was going to hear what he told me, he then said quietly, "Foot care, foot care is the key."
Absolutely true.
Me running in Death Valley on a portion of the Badwater Ultra Marathon course. 135 miles from Badwater Death Valley to the portal on Mt Whitney. Starts at 282 below sea level and finishes at 8,300 elevation. No, I did not run the entire 135 miles.