One thing I have learned the most about writing fantasy is to have patience. Yes, patience.
I started writing my fantasy story after lots of years of building the world. I had to know who was going to be involved, what rank they held (if any) and what their purpose was. If I told you it only took me a few short weeks to do all of this, I'd be a better storyteller.
In truth, I worked on my world for over ten years before writing down the first part of the story. Most of the planning was put to paper, but a greater portion was in my head. I had to visualize a great deal. The first story that came out, wasn't quite what I expected, but by this time I was in such a rush to get it out to the public, I didn't stop to take the time to plan the rest of it.
Even after the first story was done and I went through the whole, “What Do I Do Now” period, my brain was working. It changed the story, shifted things a bit and I ended up writing “Adversarius”. See, that first story was really just my rough draft. I wouldn't have written Adversarius had I not fouled up that first book.
Patience would have helped me determine this eventually, I suppose. And even though the first book is now done and in the hands of readers, I'm still being a bit overly cautious and patient to get book two done. There are so many things happening in my world that I have to make sure I keep a good eye on it all.
Once it is done, though, I need to practice patience all over again to get it polished up and ready for the readers again. Half the time, I'm so impatient, I skip over things. I won't do that again and hopefully book two will be better than the first.
So have patience when building your world and writing the story. There are lots of details to include and you don't want to skip over any of them.