I'm not sure if this answers your question, but to help determine how much you need to eat versus what you expend, you can track them using one of the many online calorie counters like FitDay, Carolie King or Calorie-Count.
They aren't precise and all have their shortcomings. But they will give you an idea of what you're using in energy and how much to eat to replace it. The key is eating a little something (bagel, bowl of oatmeal, energy bars) at least an hour prior to the long runs and then eat afterward to bring up your blood glucose levels and the return of your strength. Once you get up there in the mileage, you may need a little something during the runs to sustain your running level. That's where the Gus and Shot Blox and Gummi Bears and pretzels come in handy.
There is no hardfast rule on how much to eat, and a lot of it is trial and error during the training. Good luck finding the right combination.
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