From Christine Luff,
Your Guide to Running & Jogging.
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A. When you run for under 90 minutes, most of your energy comes from stored muscle glycogen. If you're running for longer than 90 minutes, the sugar in your blood and liver glycogen become more important because your stored muscle glycogen gets depleted. Fueling with carbs during your longer runs will prevent you from running out of energy and help boost your performance.

One way to get carbs on the run is through sports drinks. Solid foods can be tolerated, but they need to be small and easy to digest. There are numerous products on the market, such as gels, bars, and even jelly sports beans, designed for long-distance runners to eat on the run. Some runners prefer to eat pretzels or sugary candy such as gummy bears or candy corn. Start experimenting with different foods, gels, and bars on your long runs to see what you prefer.

So how much do you need to eat on the run? A basic rule of thumb is that you should be taking in about 100 calories after about an hour of running and then another 100 calories every 40-45 minutes after that. You may need more depending on your size and speed, so make sure you carry an extra one or two gels (or other food). If you feel hungry or low on energy, you can definitely consume calories "off-schedule".