I'd say make a rough map and do it fairly quick and easy. Then write your book. Then see how well the map fits the book and change accordingly. Sometimes the storyline requires more or less time to get from point A to point B and the map will change. Sometimes the story will require certain things to be in place that are not there currently so the map will change. It's a fact of life that any map that comes before the story will change. Being more of an artist and less of a writer the map comes first for me so every map I make I start stories in my head and that's where names and unique features come up and get put in.

Also look at it this way, every time I do a commission I ask for a rough sketch. That means the writer has written his stuff and has a better idea of the layout of the world than I do and probably has that idea roughly in his head while writing and that image changes to fit his story as he writes. Since you're the writer and artist do a simple map sketch first and then write. Once you have a section nailed down or a chapter written then go back to the map and flesh it out some more in detail.

In the end either way works just fine.