Today I came into the office with the goal of writing one section of the grant application. It turned out that this only took me about 25 minutes, so the rest of the morning I'll spend thinking and writing about the other two main sections so that I'll be ready to do another section tomorrow.
Working on a single, limited and discrete section of a daunting project in a given period of time can be a highly effective technique. Concentrate all your energy on one very small task. It's like applying the entire force of your body to unscrew a particularly tight screw. Maybe your fingers hands and wrists are not strong enough, but what about your arms shoulders and hips?
Books are broken up into chapters, chapters unless they are extremely short will have subdivisions. You might think of a day's work as writing a page, leading to a week for a particular 7-8 page subdivision of the 30-page chapter.