One cheat of the AI's is the "run around till the battle timer runs out trick". Essentially the AI keeps its distance from all your troops until the battle timer expires or, if there is no battle timer, till you quit the game.

This is particularly bad if your force size is limited and/or lacks fast cavalry units - they can keep your force running after them for hours with very little chance of capturing them. You often can use a unit to chase them while letting another rest to gain strength, but, if their unit is fast, then your situation is rather dire if your alternative unit isn't equally fast (or slightly less fast considering that the foes are tired).

In the end, you often have to use an ambush to nab them, but, if you only have one unit, this is not possible.