It IS possible to break their armies up by running near them, fleeing a bit and repeating this process for a few more times until one or two lords decide to go hot on your heels. I do it often too! This is only good if there is a good distance between their force and your said city though, otherwise your city might have been taken over by the time you're done with a few of them.
Still, defeating a 2400-strong force is no easy task no matter how you do it. I'd really recommend letting them take the city then re-taking it immediately if they do not garrison too many of their troops there. If they do, well, consider your taking of that city untimely. Better luck next time!