Er, I haven't got that far but I think you answered the question yourself. I am level 18 and I've just signed the peace treaty with the dwarves. Now I am planning on going back into Kordor to clean everything out, same as I did in Darion and Western Islands. I should be level 20 before I even hit the Elven continent.
This is the good thing about playing on Hard, by the way. You HAVE to look for the enemies you can defeat, or you'll get owned. So go back and clean out the earlier continents.