For some people, the idea never leaves the thought process, and for others, they jump right in and start writing. If you're thinking about it more than doing it, your heart may not be into it. Writing requires a lot of time and dedication, so if you aren't truly inspired, then chances are, it may not be for you. And hey, there's nothing wrong with that!
Then again, not everyone has to write a book either. Some people are more inclined to work on a blog, while others might want to write in a journal, create poetry or even come up with songs. It depends on your style and interest. For me, I can't imagine ever writing a short story. I also can't envision writing a song or poem. I'm not interested in all the research required to write a non-fiction book, but fiction? Now that's my baby!
Of course, you have to also look at why you're doing it. If your only interest is writing a best seller and becoming rich and world-famous, you might want to change your plan. Not to say that this isn't possible, but it rarely happens overnight. Writing books is like starting a business. You probably will put a lot of work into it before you start seeing results as you slowly build an audience. Then again, if you're writing because you can't imagine not writing, then you are probably on the right path.
As I said, writing can take many forms. You don't necessarily have to write a book, but if you want to, then don't overthink it. Just do it.