I’m talking about pain/annoyance threshold, not general neurochemistry.

> pain/annoyance

Can be mitigated via drugs and increasing thresholds with, as you did, training, which results in decreased stress hormone levels from lower forms of physiological and psychological stress, better lactate metabolisms (not sure it's an actual metabolism, but better fitness results in higher storage capacities for lactate and quicker recovery), etc.

And endurance literally means your body and brain can suffer/sustain higher levels of stress due to fatigue and damage because the mechanisms causing "overflow" and "breakdown", "total collapse", even if it's just for the sake of reducing subsequent damage, are balanced by their biochemical counterparts and, again, higher storage capacities for pretty much everything body and brain produce while generating, transforming, exerting energy and all the exhaust ...