I am not sure if I read about it somewhere, but when I felt it I searched for it. It’s a state of euphoric mind, a state of happiness that cannot be explained, a state of euphoric relaxed body. It is called “Runner’s High”.
It is different from the sense of accomplishment you get when you stick to schedule or overachieve.
When does it kick in ? Not sure, in my case I usually feel it at the end of the run and afterwards. One day it lasted for hours.
Remember not all runs will give you this state of euphoria, but you need to push in your run routine in order to someday it’s gonna hit you and blow your mind.