I have an interesting idea.
Make troops with mercenary settings gain a large amount of morale from looting. So you can march your troops a long way if you loot people on the way. Especially people that had previously been neutral to you, and even sympathetic to your cause (but, hey, your men want their treasure or their women, and they don't want to wait until you get to your destination). . . .
Well, it worked for the crusades.
(Note: the above suggestion is intended to be tongue firmly in cheek, although it would be pretty cool if mercenaries allowed you to go really far, but you would sometimes randomly loot your allies along the way or something. It's historic!)