I have heard that doing cardio on an empty stomach when you first wake up will burn a lot of fat as compared to doing cardio a couple hours after your first meal.

I was curious to know if this was true or not, and wether or not cardio on an empty stomach would also burn a lot of muscle?

I have heard that doing cardio after a workout burns more muscle than fat?

And what about on leg days and a day or two after? Would it be bad to do cardio because it wouldn't allow my legs to fully heal or what?

Also, how many days should I do cardio? I have heard that three days of 20 minutes or more each day is enough but I am not sure on this.

And, how much more effective is HIIT cardio then running in the morning, after a workout, or any other time?

I need to do cardio for football conditioning, so I figured I might as well try and burn the most fat and least amount of muscle.

What are your thoughts on this?

(Sorry for all the questions)