"The mathematics in Rapoport's paper are, frankly, beyond most of us (me, anyway). But he has developed an online calculator at endurancecalculator.com to simplify the process. After all, the main goal behind his work is to make "computational marathoning" (my words, not his) accessible to real-world marathoners. He wants runners to understand that The Wall is not an inevitability; it only results from inexact science."
I have a theory that if you have a fat-optimized metabolism you may be able to adapt a different fueling strategy. Our hunter-gatherer ancestors probably didn't have to depend on finding a nice source of carbs every time they set out to run down dinner, and bonking in the middle of the savannah would clearly not be a survival trait. Unfortunately I have no evidence to support my suppostion, since most modern competitive runners have adopted a carb-based fueling strategy.
But this issue may well highlight the line between our persistence-hunting ancestors and a modern competitive runner. It seems to me that if there is a case where "Chronic Cardio" would start to become a problem to your health, this is likely over the wrong side of that line.
I'm not endorsing the notion of Chronic Cardio, btw, and I hope to do a post soon explaining my problems with that theory.