I know what to write. I've spent years working on this series. Starting to write the story proper--aye, that's the trick. There are too many distractions, too many side plays, when you spend decades building a story.
Not to mention that I am an horrid, slow typist/keyboardist.
I have to remember, that I'm giving myself permission to screw around on the first draft. the words will start to flow, at some point. I know that from long experience, and from much writing. The hardest point is the starting gate, with a nebulous end in sight.