but that's not realistic either. Sieges always took months, so the army would always be resupplied by nearby friendly cities. They would never try to live of the land, that's impossible.

Maybe in combination with your idea there could be supply trains looking simular to the trade caravans you see on the RTW campaignmap, going from the nearest friendly city to your army, using the shortest road. The enemy can then blockade these roads to stop these supply trains, and the longer the distance, the more expensive it becomes to maintain the supply train. The supply train can be switched on and off for each army, so the player has the choice to let small armies try to live off the land (which is the default option).

just thinking out of the box here :)