Congrats!
I haven't written fiction yet, but I found two things worked very well when I was in a content-cranking zone...
(1) Scrivener. Great program, I bought it on an whim from AppSumo and it was and is awesome.
(2) Having a script or a guide BEFORE YOU START - which is what you underscore with this post. Yes, you can change as you go, but your book is written in the table of contents (at least with non-fiction).
Cheers, mate, and congrats!
Thanks! I used Scrivener for this work and others, and once you learn its foibles it quickly becomes indispensable for writing long-form work.
meep