Q: Fifteen minutes isn't enough time to get into something.

(That's not a question.)

But it's true that 15 minutes isn't a lot of time. The point isn't to finish something--it's to start. This is PLENTY of time to start writing your book. If you only write one page, you are now "writing your book." You can do more if you like, but now you've started. Congratulations! You're a writer. It counts.