How to write 500 pages? One page at a time.

[S]uddenly the United States became huge beyond belief and impossible ever to cross. . . . It was like starting to write a novel. When I face the desolate impossibility of writing five hundred pages a sick sense of failure falls on me and I know I can never do it. This happens every time. Then gradually I write one page and then another. One day's work is all I can permit myself to contemplate and I eliminate the possibility of ever finishing. 

John Steinbeck, Travels with Charley in Search of America  (New York: Penguin Books, 1986), p. 23 (orig. published 1962)