Voted: "Do all of these but try to divide time equally."
The book writeing not easy... if you want building a full world. ^.^
I have four world and novel in my mind. Already started at write... but i would have liked make illustrations and don't felt so free myself. : )
Began develop myself, in some sector, line by line - step by step. ^.^ Well, now better. But, this is "neverending story"...
A piece of advice: Do don't it for a long time. (I have been about planning and developing for ten years my fantasy and sci-fi book... but not yet ready a book or version 1.0)
I mean.. if you have one base version... you been one step further forward. ^.^
(This is just an idea, opinion...) I hope helped this.