How do I increase my piano skill level?

Right now I am in the middle of learning the 1st movement of moonlight sonata, I'm almost done! What I want to know is how much harder is Rachmaninoff's Prelude in C sharp minor (Bells of Moscow). I have printer the first few pages and honestly, it looks terrifying. Is there an intermediary piece that I could learn to move me from "moonlight level" to "bells of moscow level"