I was finishing up on the last chapter of "Colin and the Little Black Box," and realized that I had been tired. I thought it had been finished. Beware writing when you're tired. The mind may be all right, but sometimes what you are thinking just doesn't make it down onto the page.
This brings me to the point of this post: pacing. Everybody writes at a different pace, and that pace should be suited to your personality type and the environment you are writing in. For me, I write in limited, but very productive time blocks of about an hour. Then I get away from the computer, go for walks, think about what I'm writing and where the characters are going. It lets my mind process what I'm trying to do. You see, I'm a deep thinking person, but not very quick on the pick up, so I like to do a lot of thinking before I write. As a result, when I sit down to write, the fingers fly and I slip into the zone.
So, what I'm saying is find what works for you and to do that you may have to experiment a bit.