For some people, the idea never leaves the thought process and for others, they jump right in and start writing. Chances are if you are thinking about it more than doing it, your heart may not really be into it. Writing requires a lot of time and dedication so if you aren't truly inspired than chances are, it may not be for you.
Having said that, not everyone has to write a book either. Some people are more inclined to work on a blog, while others might simply want to write in a journal, create poetry or even come up with songs. It really depends on your style and interest. For me, I can't imagine ever writing a short story and 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 you think that writing a book automatically makes you a world-famous millionaire, think again. Not to say that this isn't possible but it rarely happens overnight. In fact, writing books is like starting a business. You probably are going to 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.
At the end of the day, you need to love. If you're passionate about it, then you need to follow your dream.