So, anytime you exercise, your body burns sugar (i.e blood sugar) to meet the sudden need for excess energy. Any diabetic will tell you having too much blood sugar is a bad thing, so you only have so much blood sugar in your system (say a few miles worth of running) at any one time. Now, your body, not wanting to waste energy reserves converts sugars into fats when your blood sugar is at a normal level, which, in periods in which you're beginning to expend your blood sugar it begins to cash in. However the process that breaks down fat into energy is much slower than the process that converts sugar into energy. As a result, if your energy demand is high (marathon) your body simply can't break down fats into energy fast enough to keep up. Eventually you will run out of blood sugar and be physically incapable of maintaining your pace by burning only fat. A quick shot of simple carbs/sugars allows your body to bridge the gap between the energy its getting from fat reserves and how much energy you're needing.
Thats why you only need 60 cal/hour, whereas you're burning 1000+/hour. It supplements your existing sugar/fat reserves to create a sustainable pace of energy consumption.
