When you are at rest and not exercising at all, your muscles are burning both fat and carbs for fuel at the same time. At rest, the majority of what you burn comes from fat - and depending on the sources you read, it can be somewhere around 70 %+/- that comes from fat. With bulk of the remainder coming from carbs. The thing to remember is that, to burn fat, you have to have oxygen. But, you can burn carbs with or without oxygen. So when you exercise, your heart rate increases and your breathing becomes more strenuous ( i.e than compared to when you're at rest ) due to an increasing lack of oxygen. In most cases, the harder you exercise, the more out of breath you'll be, so the more in oxygen debt you'll be, so the more you;ll be relying on carbs instead of fat ( because fat needs oxygen to burn ) for fuel.
So, if I were to exercise for 20-30 min of cardio and barely getting out of breath, I'd be burning mostly fat as fuel. If I were to exercise for 20-30 min of cardio ' flat out ' , as hard as I could go, out of breath the whole time, I'd be relying more on carbs for fuel than fat. So, if your want to optimize the amount of carbs or fat you burn during exercise itself, I'd focus more on the intensity of the cardio than the duration of the cardio.
That said, I wouldn't focus much at all on the relative amount of carbs or fat you burn during exercise itself. Instead, focus on ' total calories ' you expend as a result of exercise.
If your goal is to lose fat via exercise, the primary aim is not to determine how much fat is burned vs carbs during exercise itself IMO. So, if it were me, I'd forget about things like ' fat burning zones ' and ' cardio zones ' etc.
Rather, the goal is to try and burn as many ' total calories ' as a direct result of exercise ( both during and afterward exercise ) as you can. So - all other things being equal - running at 65% MHR for 1 hour will burn more calories than running at 65% MHR for 20-30 min. And for his example, ( putting muscle loss concerns aside for the moment ) that means you will lose fat quicker by running for 1 hour vs 20-30 min at 65% MHR. And, if you are going at a moderate to low intensity and for 1 hour and you have been eating properly every day throughout the the week ( meaning you should always have you glycogen stores topped up ) the risk of significant muscle loss due to cardio should be very low and nothing I'd worry about IMO. It really becomes more an issue if the cardio intensity is high enough and the intensity is sustained long enough so you end up significantly depleting your glycogen stores - prompting your body to turn to muscle as a source of energy.
However, you may also want to do HIIT. And, doing 20 minutes of HIIT may only burn the same number or less calories ( in 20 minutes ) than say 30 minutes at 65% MHR. But, because HIIT does a better job of burning calories after exercise, you may end up burning more ' total calories ' ( total = calories burned during and after exercise ) by doing 20 minutes of HIIT than say 30 minutes of slow & steady cardio. And since the key to losing fat via exercise is to burn as many ' total calories ' as you can, HIIT may be the better choice even though it takes only 20 minutes and burns mostly carbs ( not fat ) as fuel.
The point being , forget about how much you burn as fat or carbs during the exercise session itself ( i.e the " fat burning zone ' ) and focus more on optimizing the ' total calories ' you will burn as a result of exercise.
