When doing cardio do you burn muscle before fat?

No. Your body prefers to burn the macronutrients in the following order: carbohydrates, fats, and proteins. Therefore, if you have carbohydrates available in your bloodstream when you do cardio that is what you will burn first. When digested, all carbohydrates become sugar. This is why (if you are using cardio to lose body fat) the best time to do it is before breakfast when you have been fasting overnight. As your body depletes the glucose in your bloodstream after about 20 minutes, it will turn to burning fat, which is what you want.