That thing is:
The body can be trained to do whatever your mind has the will to train it to do.
This seems particularly applicable to the new style of running I've been trying. It's more painful (sometimes much more painful) but it seems to yield better progress overall.
From a strict pain/utility perspective, a slower continuous pace might seem to be optimal, but for a longer term perspective of no slowly degrading performance over time, I think it makes sense to train for the pain. That way I'll always know that I'm pushing things to the practical limit and won't slide down a slippery slope of complacency.
Only by having the will to train through the pain would I get to a better overall solution.