If you were to extrapolate Tolkein's world then it would eventually wrap around, all worlds are globes. So I would say map out the entire world just to do it but then restrict yourself to one continent and maybe a few islands. This way you always have some room to expand if you need to and also provides the base for future stories. When it comes to showing maps for a book then just show the one continent. You can leave the other continents as vague shapes for now. It's fine for you to know more than the audience and you should, you're the stroyteller so always leave something in reserve.

The way I look at it is that on one continent you can have say three or four races of people (elves, dwarves, etc) and with the whole wide world left out there to explore you can put in other races (like orcs, halflings, and goblins) or others types of people (say like Asian, or African, or Polynesian). The main thing is to come up with some sort of superstition as to why no one has ever gone beyond the "known world" sort of like Christopher Columbus...he didn't fall off the edge but that's what people believed. They also believed that Atlantis is buried somewhere out there too hence the name for the ocean as Atlantic. Make up a story, the more outlandish the better.