True about the fat / glycogen, although that's short term. I'm not confident, but from what I've read on it, our body is quite good at converting and allocating calories. So that if you spend 1 hour zone 2 burning mostly fat but 600 cals, vs. 1 hour zone 3 burning glycogen but 800 cals, when you eat 600 cals you'll still backfill the fat/glycogen, but zone 3 would still have been better for weight loss because of the increased calorie expenditure. So that zone 2 is better for fat loss very short term, but not necessarily long term unless you're also putting in more miles than you would higher zones. But regardless of the nitty-gritty you're doing the right thing focusing on zone 2 because it almost assuredly helps you run more and overall burn more. Most people do way too little because the higher zones feel awful to them.