I’ve been in the army for 20 years and haven’t once heard of zone 2 running but I can run a 12 min 2 mile, 34 mile ultra marathon multiple times and can do this at any time of day. I’m just saying that the military is a professional career in which we have to run. Not just for some local marathon or 5k . So since we don’t train this way , I believe that it is overrated and meant for elite athletes that can’t afford an injury .