I was never a good runner. 12 miles at 8+ minute miles but I recall the sensation you refer to......you just get in the zone and its lovely.You will recall I quit running in late 2018 because of arthritis in my big toe making it excruciating. I too miss it. The first mile or so was a panting, chest thumping agony then suddenly, BOOM! I was in the zone, legs going like clockwork, brain relaxing on a lilo in a warm swimming pool of endorphins.