Compelling Fantasy Mythology

The main problem with crafting detailed mythologies for your setting is that you could have just set the story in those instead. That's why you should include a brief conversation along the following lines:

"'This reminds me of that possibly legendary story.'
'I know just the one you mean, and of course there is no need to elaborate.' He looked at the camera and winked."

Addendum

That old sailor is a sea god, but you won't find that out for a few books, or maybe in an interview.

Number One Reason to Include a Map in Your Book

Your characters can flip to it if they get lost. Nobody wants to wait fifty pages while they try to remember what they heard about stars once from an old sailor.