Is this because I'm not accustomed to writing so much?
Probably, if you haven't wrote much(Writ much? Writed much? Something like that, you get the idea) before, then it is understandable that you can't just sit down and write out a dozen pages of stuff. I can only do 3-4 pages in one sitting before I take a break. And thats on a good day with no distractions and a good motivator.
What should I do?
Well, the obvious answer is to practice. I'd start there, keep writing as much as you feel you can do, and then another paragraph or two more. When you come back later, edit those last paragraphs if you feel like it needs it and continue.
If that doesn't help much, then maybe try a different practice strategy. Other than that I'm not entirely sure what would help.