Before my knee told me we wouldn’t be running anymore — I would find this as well. Not in the beginning — as you said — too much going on & your body acclimating. But in the middle — once I hit anaerobic running — I could breathe easier & I could feel the rhythm of the pace. My mind would just focus on the sound of my feet. I know what you’re talking about. It never was about the runner’s high — it was about the runner’s zen.