"In simple terms, lactic acid fermentation is like one thing of food (a peice of bread, fruit, etc.), while respiration is a full meal. If were dying of hunger, you would probably not take the time to make the full meal, you'd just scarf down a peice of bread or something. Certain muscles may start "dying of hunger" at any point, especially if your not using efficient bio-mechanics. You probably wouldn't have every-muscle involved "dying of hunger" at the same time until the end of a long run, but periodically individual muscles, or even just groups of cells would get too "hungry" and switch to fermentation, until they're "full" enough to take the time to make a whole meal. You can actually do fermentation and repiration in the same cell at the same time, but lets not get into that."