A Writing Rush

So apart from the internship (which I will not say much about apart from that it took some time to get used to working in an office with a lot of other people around me after working at home for so long, but which is going very well and which I am enjoying immensely) I am also writing my MA dissertation. This is the final big piece of independent research I am required to do before I can graduate, and it has to be about 15,000 words. I'd known since last year, when I took a course on the subject, that I wanted to write about Jane Austen, and already arranged my proposal and background chapter before going to the US.
But then, when I came back, the struggle began. How to start. I think this is the biggest problem most people who have to do a really big assignment that will span several months face, and for me, it was no different. I was planning to start the Friday after we came back, but no, to much other stuff to do; the Monday then, no, to many appointments; the Tuesday, no, then I had a meeting with supervisor and could still put it off until finally, last Friday, I actually started writing. This feels like it was weeks ago, so I am surprised to think that I've only been officially writing for 5 days. By that time, mind you, I was completely freaking out, already behind on my own schedule by 2 weeks, and convinced I would never finish the thing.
I know a lot of people are freaking out about their dissertations right now, as it is already October and we have to be done before February, but as usual, I should not have bothered. About an hour ago, I finished my first chapter. It's 2,500 words, it compares the two novels I wanted to use in depth, using 3 primary and 8 secondary sources. I have no idea how I do this, but somehow, whenever I start writing, things just work. I get into this zone of writing, which usually lasts about 3 hours, and at the other end I'm halfway there. Add another day like that and most of the chapter is there, and then a final day to make some changes, put in some extra footnotes, and check the last things, and done.
This does not mean that the chapter is 100% done, of course, I could keep adding more onto it until my entire dissertation consists of just this chapter (which I won't do, obviously, unless my supervisor wants me to add something), I still have to re-read it to see if I don't want to change any sentences (I haven't even spell-checked it) and also, once my supervisor has looked at it she may well ask me to rewrite the whole thing. But most of it is there. Again, I have no idea how I do this or why this works for me. It's highly efficient, as I'm done after only a couple of hours work each day, but this somehow bothers me and I keep sitting behind my computer thinking up all the things I could be doing (right now, I'm already writing stuff for the second chapter, the easy bits which don't need that much thinking over) just because I feel like I should be doing more. I know some people are jealous of me because it seems to come so easily, but they may be forgetting that I'd already read 15 books on this subject (plus the 6 novels I'm using) before I started writing, and that I've been thinking about this stuff for 3 months now. So it's not that strange that things easily form connections and make sense in my head, but also, it's not as if it's effortless, because I am completely drained after a day's work.
Still, it feels like I should be doing more, as if it's coming too easy to me, as if what I've written can never be good enough. Over the years, I've tried to ignore that voice and just handed things in a week before the deadline only to stop myself from fidgeting about it when it was really done. I think I should do the same with my dissertation, even though it feels really weird to stop working before 15:00 when you still have so much to do. But if I can do that amount in a really short time period, maybe I should start enjoying all the time that is left over a bit more, instead of worrying. Ah, if only...