There’s nothing like getting lost in creating your exciting odyssey. It’s a great escape, not just for readers. But first you need the whole story. I suggest trying something smallish first and saving your magnus opus for later, so you can experience the whole lifecycle quickly to see if it’s really for you. Writing’s not just rewriting (as someone said), there’s also production and online marketing too which can be half or more of the effort on a title. If you’re going to use professional editing, branding and production it can get expensive too. Read widely and learn the craft as you go, incorporating it into your book. There is always a strong desire to just get it done and out, to see your name on the cover. I think it’s better to take your time and don’t rush. Get it right. Finally, just start. Go! It won’t write itself.