It's hard to tell on marathon. Usually the AI tends to be really weak early on, because of how long it takes to rebuild the military units that they lose, but if a Civ manages to become a runaway and get enough land to swarm you, they tend to become ridiculous later on. On smaller maps, that doesn't really happen - in fact, on small marathon maps it's quite possible to just wipe out (or cripple) the whole world before entering the Renaissance-era, even on Deity.
So overall it depends very much on your playstyle. If you're aggressive, then it'll probably be easier, if you plan on playing passively, then you'll most likely have a harder time.