Okay, this is the part where Tacky gets snarky.
Dude, Martin, WRITE!
Come on. For crying out loud, man, why can't you finish the freakin' book?* What is it with these people who start an enormous, epic-scale thing, thinking "Hey, no big, I've got it all taken care of", and then it gets out of control and messes them up, just like it messed up all the other authors? Criminey. OUTLINE. PLOT. THEN WRITE. If you've got writer's block, have a drink or smoke something mellow-inducing and get over it. That's what you get paid for.
I'm working full-time, and I can crank out a book every other year without the satisfaction of seeing any of them get published. You've got three hugely successful books and slavering fans. How can you NOT have finished this thing yet?
* - I am not aware of any tragedy or personal illness or anything that would keep Martin from writing the book. If he has gone through something massive and heavy and bad, of course, I am a pinhead who should just shut up.