In a nutshell, depending on intensity, yes, you are mostly burning carbs earlier in your workout, however, does it really matter? Pretty much if you expend your carbs in your workout it means your insulin sensitivety increases due the bodys desire for carbs and also, carbs = fat when their unburned, so in the end its all about the amount of calories burned more so then burning fat.
Now does that mean low intensity is bad? No, both have their advantages however, dont make the workout super easy just because your burning 70% fat 30% carbs at lower intensity as opposed to 50% fat 50% carbs at higher intensity why? Lets do the math
High intensity 20 mins: Calories expended 300 150 fat 150 carbs
Low intensity 40 mins: Calories expended 250 175 fat 75 carbs
So in the end, your spending half the time and expending more calories, however, not as much as fat, But your also increasing your insulin sensitivety massively and also if executed at a high enough intensity can cause a afterburn effect which keeps your metabolic rate up.
Now does that mean high intensity is better then low intensity? No, but it is something to consider
