The more I think about it, the more I like the "add 5' required to stand up for each adjacent enemy." Try lying prone right now -- not just lying down casually, but like you just got knocked flat on your face, which is how combatants typically end up prone in D&D. Now try getting up and running to the other side of the room. Imagine there are burly men with swords next to you while you do so. Do you feel how vulnerable you are while standing up? You couldn't just stand back up like nothing happened; you'd have to be really, really careful.
I still think it should be possible to at least stand up regardless of how many enemies are adjacent to you, though.
I still think it should be possible to at least stand up regardless of how many enemies are adjacent to you, though.