When I first decided to write a book, I never thought I would accomplish my goal. It was daunting to imagine writing so many pages. The approach, however, I took was to break it down into bites. It made it not just appear more manageable (and possible), but it indeed it was more manageable (and possible).
Also, remember that it doesn't have to be perfect the first time around. I began with word "throw-up" and just kept revising. Enjoy it. It's worth it when you have an end product that is your creativity--a piece of you. Whether fiction or non-fiction, it's always a piece of you.
The best advice…just sit down, write and have fun!