One of the great things about NaNo is, it shows what can be done in a committed amount of time. To write 1,667 words each day for thirty days amounts to 50,000 by the end of the month. This relies upon the habit of sitting down and writing every day for thirty days. Once you miss a day or two and rerun the numbers you can see how the objective creeps out of range.
Word count as a metric is useful for measuring raw production. If a book is two hundred pages, and I write only one page a day, a rough timeline appears. I’m convinced now the key to productivity is not in the duration of the writing session but in the iterations. I produce way more in four, thirty-minute sessions than in a single multi-hour session.
Writing daily also helps me keep the project closer to the forefront of my mind rather than just something nagging at me from the back.