Writing a fantasy novel isn't as easy as writing other genres such as horror or mystery. Other than science fiction, these  genres are mostly grounded in reality. Sure, you can introduce the occasional moody vampire, but its still just a small town in Alaska.

They are hit by gravity the same way we are and put on their pants just like anyone else. This isn't so in a fantasy novel. You are creating new lands where trees can grow upside down and magic rules the land. Rules is the operative word here. Before you sit down and write your fantasy story, you need to create the rules for the land and its magic.

What does it require to create a spell? Does it have some effect on the caster? Are they tired, injured or debilitated? Can only certain types of people cast certain types of spells? These are all factors that weigh into your great fantasy story.

Once you've got those aspects down, then you need to weave it into your story. It isn't easy. You may have your hero doing something that completely contradicts the rules you've set forth. Your only choices are to use some type deus ex machina to give him immunity from your rules or rewrite the rules. A minor rule change half way through the story could have a ripple effect throughout the story requiring rewrites.

Fantasy authors are a rare breed because they are like great chess masters. The plan everything out ahead before any words are even put on the page.