You can interrupt this quote by Richard Branson a couple of ways.
I like to think that it reminds me that every project starts with small steps. If you’re not willing to put time and effort into those (for example, research for your book, fleshing out your character), then you’ll probably fail at the big step (getting the book actually written). Everything worthwhile means you have to devote time to it.
It also reminds me that every project needs to be broken down into the sum of its parts. Your overall goal should read I’m going to write a book, but take time to write down each and every step you’ll need to take to get there.