Running lows

I just finished an 8 mile "run", and stayed above 67 the whole time. I feel alright about it, but it brought up a question for me. 

For those of you who run (or do other endurance-y exercise) how do you deal with lows? I've heard a lot about ways folks try to prevent lows while running, but we all know they still happen. When they do, what's your response? What do you eat? Do you keep running or wait it out? Do you throw in the towel and walk home? Does it depend how long your run is? So many questions!

I'm trying to figure out how to be safe and in shape, and want your input!