First off, I did not mean to suggest that you do resistance training WHILE you fast. I think you should be doing it the day before you fast as part of your regular routine.

Also, any kind of weight loss will come from both adipose tissue and muscle -- it doesn't matter if you fast, or if you count calories and eat smaller meals, or if you go keto. If the weight is coming off, it comes from both places. And the studies I have read (and summaries of those studies I have heard talked about) indicate that regular resistance exercise is superior to eating lots of protein in terms of maintaining muscle mass during weight loss. It seems to me that eating protein would cause your body to use that (instead of your muscles), but I'm just reporting on what the counterintuitive studies say. Don't shoot the messenger.

It might be that muscle tissue starts being used after 6 hours (if you are correct) but if so then it happens every night while you sleep.

It usually takes from 18-48 hours of fasting before autophagy kicks in, the process of desperately looking for protein by consuming broken down organelles inside of cells. Autophagy scavenges protein from cellular organelles, so if you aren't doing it regularly, you'll still be getting some protein from that. Before that there are a lot of substrates that can be used (lactate, pyruvate, alanine, oxaloacetic acid, free amino acids in your blood, liver, etc). Also when fasting your testosterone shoots up 1300% and human growth hormone by 400%.