They've got to calculate everything for each turn, don't they. Pretty much what the above poster said, but I'd play a bit first, to consolidate your territory and get some money at least, otherwise you might find yourself faced with anything later on (excellent Macedonian stacks, for instance, instead of the Red Menace - for some reason in several campaigns of mine I saw a MacAttack in that direction, they were even pwning the Romans once).

The other way would include modding the game to have a full late campaign, personally I don't think wouldn't be able to do it.