If you have never read Middlesex by Jeffrey Eugenides then as a writer you are missing out on a real lesson of what it means to write a beautiful book. The story is dynamic, travels and is compelling and the writing is beautifully done.
While reading the interview of him in the Paris Review I came upon the following quote early in:
"I do. I try to write every day. I start around ten in the morning and write until dinnertime, most days. Sometimes it’s not productive, and there’s a lot of downtime. Sometimes I fall asleep in my chair, but I feel that if I’m in the room all day, something’s going to get done. I treat it like a desk job."
Reading his writing it's hard to imagine he sees his work as merely a desk job, but there is a lot of truth to the notion that just showing up is little more than half the battle. Most folks work at their jobs and write in the moments between everything else, which is fine, but if you can find a way to make it a daily habit, even if it's different times each day, the progress you'll make is considerable.
You also should be very forgiving of yourself if you don't show up. We all fail to make the daily appearance. Forgive and do your best to sit down and write just one true sentence. If you have time, write another.