I've managed to hack out about 20% while working, raising kids, managing life, etc, etc. But I know once I can concentrate fully on it...much of what is already there will change.
I always do that. I start a book, get going, then at about 20% realize I'm either not happy or I see where we're going in a whole new light and end up scrapping a large chunk of what's already down. (Or at least it gets significantly rewritten!)
Sometimes those first 60 pages are just warm up or "getting out of my own way" to quote someone I read somewhere...I don't remember who. I wonder if I didn't stop writing while working if I would still need to do these throw away exercises--like an athlete whose stopped training. I have to stretch A LOT or else I'll blow something.