In my experience, significant work can only be done piecemeal. I'd estimate it took George RR Martin around 600 hours of writing to produce Game of Throne: there's no way he did that in a single sitting. Further, I'd be buttons to bullion that it went through at least three drafts (probably closely to 6), each with a respectable break in between. We're looking at around a year and a half of full time work. Trying to mimic that only when you have large chunks of time just wont work.
The key to quality isn't having a huge chunk of time to work on things, but rather working on things consistently and going through multiple drafts. I'd be greatly surprised if there's a book on the market that had only one draft. Indeed, even just two would be incredibly amazing. This means, the first draft is going to be crap when compared to the final version. As such, there's little good in worrying about getting it absolutely right the first time. The biggest challenge, rather, is getting the thing hammered out and down on paper. THEN, when you know what you are working with, can you start crafting it into something of quality.
In short, now is not the time to worry about time, you don't have the time.
As for productivity, one of the best pieces of advice I've heard (once I was in a place to understand it) was that one has to treat writing like a job. You can't get by if you skirt off on your job's duties, and you can't get by if you skirt off on your writing. It doesn't matter if you feel like going to your normal job and working, you still do it. And it doesn't matter if you feel "in the mood" to write or not, you still sit down and write. If you treat it like a hobby, it will ever only be a hobby. If you treat it like a career, though, then you just might make it one.
But as I said, sometimes people simply don't have the time for even scrounging up a half hour here or 15 minutes there. I spent much of 2010-2011 in that state. I get it. But the sentiment here is finding every ounce of energy you can spare and putting that towards your project.
EDIT: I noticed that you indicated that you essentially have to write yourself into the story to really get going. A lot of writers are like that. However, if you write consistently (as in, every day at noon, sharp, you write for half an hour), you will be able to get into the story much faster. This takes time, though, to develop the needed "muscles" and habits. Furthermore, thinking about the story and what you want to accomplish that day really helps me hit the ground running. I don't have the narrative figured out, of course, but just knowing that "This character will be here, go there to meet with that other character, have a fight, banter, then run away" makes the days writing session go so much better (and it's easier to loose myself in it and write beyond my time limit, because then I am eager to get to the rest of what I planned for that day). When I'm not thinking about the story all day, then things really drag when I write.
