Many writers have observed that it is ideal, having turned your story over in your mind, to write the first draft at one sitting, pushing on through the action to the conclusion, no matter how dissatisfied you are with this paragraph, that character, this phrasing, or that incident. There are two advantages to doing this. The first is that you are more likely to produce a coherent draft when you come to the desk in a single frame of mind, with a single vision of the whole, than when you write piecemeal, with altered ideas and moods. The second is that fast writing tends to make for fast pace in the story. It is always easier, later, to add and develop than it is to sharpen the pace.

Janet Burroway, Writing Fiction, Tenth Edit…, loc. 76