Perhaps you could do it on a 5 or 10 year basis.

5 years would give each ruler 20 game turns to play out, and if you don;t get enough players willing, 10 will give you 40.

I'd play but i don;t have the Mongol Invasion expansion, and I suck at it as well. I completed my first campaign the other day on Normal as the Mori clan. I narowly won it.