I can relate...
I've had some ideas for a while, and started writing out a script (some high level framework, some detailed dialogs) and it's both exhilarating to see your ideas grow in to something more concrete, but I agree it is also exhausting!
(Hopefully I'll be able to make it in to an AVN one day, but that means learning to pose and render...)
It's funny though, I've heard authors say before that the story wanted to go into a different direction when they were writing it than they had in their head, and I never understood that, but I'm really still in the intro section and it's already going in to directions I did not anticipate! ;)