Checkout the LHR method for increasing your fat burning. This reduces the total amount of calories needed during the training session and race. (no this doesn’t mean you burn less calories) Shorter workouts usually will not require any food during the session. Short could mean up to 13 miles depending upon your intensity and again HR. Most runners can do up to 7-9 miles without consuming calories during the run (see the drink section though). Test for yourself: Do you feel grumpy late in your run? Probably out of carbohydrates. Do you get a lot of stomach issues? Probably too many sugars or hard to digest foods. Which is all to say stop paying attention to the amount you are burning as a guide to what you take in.