A retreat is organized! And it should all be done at once. A single fighter leaving the front to seek a cleric or drink a potion is withdrawing, and hopefully a comrade will protect him while he is performing this action. A retreat on the other hand is an entire party or group admitting defeat but attempting to escape.
I've never used the rule referenced that someone fleeing has an AC of 10. Rather, fleeing characters don't get their DEX bonus and the attacker gets a +4 to hit for attacking from behind. Still, that's bad enough in most cases, so yeah, organizing your retreats can be very important.