Today I learned

I agree that mental rules add complexity, mental rules such as "If I run, I'll be seen as weird".

I also agree that the change to running not a reflex in the sense of "This pan is hot! I'll instantly move my arm!"

Without looking at the data, I'd assume the switch-to-running point is arrived at statistically. Most people have a point —or a range or a distribution of points— at which they start running. It could be that this switch-to-running point is similar to breathing: most people go from inhalation to exhalation (a point we could call switch-to-exhalation or, if reversed, switch-to-inhalation) without thinking about it, but they can also consciously control it.

The contradiction you mentioned disappears if you don't consider the switch-to-running point a reflex, and instead consider it like breathing. You can consciously hold your breath and therefore change the switch-to-exhalation point. You can also consciously walk faster and therefore change the switch-to-running point.

I agree that the exhaustion is different: different muscles are being used and they're being used differently. Maybe saying that was factually wrong. My bad. I was trying to get across the point that energy expenditure past the switch-to-running point is higher if you choose to keep walking than if you run. But the fact that there's more energy expenditure doesn't necessarily mean that you'll be more exhausted; it could be that when you run, even though you can save energy, it actually uses muscles that are not properly trained and therefore get fatigued faster than the muscles you'd use if you walked.