My advice is to skip to the part you are excited to write. There is no rule that says you have to write the beginning first. If your mind is racing about the middle, write that. The enthusiasm will show through in your writing and make it a pleasure to read (and to write).
By the time you are done with the middle, you will have a better idea of how or where to fill in the missing parts.
I do that with blog posts sometimes. The headline and opening can come later.