Each unit in the game has a Cohesion Level attached to it. Often you will experience that the opposing armies of a fierce fight (multi-day battles, for example) will rout long before being destroyed, as mighty leviathians wounded but still alive.
Wait a month or two, so that the morale of your units is restored and the replacements conscripts fill the ranks, and you will again be ready to engage the enemy. Try to follow the opponent, and you will often find that you are in almost as bad a shape as he is!
Each battle, each movement will lower your Cohesion Level, thus preventing players from long runs across the map.