The reason for this is that when exercise is not performed first thing in the morning it takes your body approximately anywhere between 20 to 30 minutes to start burning fat. This is because that it how long it takes the body to deplete its glycogen stores and switch to a fat burning environment. Therefore, it would not be efficient to perform aerobic exercise by itself at any other time during the day because you would need to perform it for 20-30 minutes just to get to the fat burning stage and for an additional 20 minutes just so that you burn fat. This would give a grand total of 50 minutes a day and in my opinion