I think about this in terms of hours not miles. At two hours if I'm not done, I eat something, cliff bar, banana, whatever. Every two hours I eat something, or drink something (With calories, not just water) or both.
I have bonked almost immediately in the past. Blood sugar was no doubt too low before I even left, worked all day, didn't eat much of anything. Working for a living makes a bit of a difference. It was extremely unpleasant, so I take precautions even though they probably aren't all necessary anymore.