It depends upon how much your world differs from what readers already know. If you were writing something along the lines of "Sword of Shannara", with most of the elements being fairly standard High Fantasy, all you really need is a map and a few notes. The rest could grow as you build your story.

(The same could be said for the reasonably set milieus of space opera fiction. A few rules on how space ships work and you're off. See the work of David Webber or James H. Schmitz for that.)

But if, say, you're trying to do something more exotic, like an alternative-history world where the First Nations of pre-Columbian North America had grown to support a higher level of technology and population and fanned out to settle Eurasia, then proceed on to Twenty First Century levels, you might have to do some serious work even figuring out what that kind of world would even look like, before you could even begin to figure out what your story was going to be.

I would suggest, if it is something like the latter, that you seriously consider putting pre-exams-cramming-time-level effort into developing it now, or what you do the rest of this month will probably include a lot of wasted effort.

I'm doing a pretty exotic setting myself, but I'm only comfortable with it because the world itself is already developed. I'm writing a prequel to an unfinished novel I dropped about eight years ago, so I finished with the world-building legwork ages ago.

(Yes, I will be returning to the other novel after this one is done, but I'll be rewriting pretty much everything.)
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Regards,
Eric Fretheim
Assistant Prep Leader, 2015 October NaNoWriMo Prep Challenge

"It is perfectly okay to write garbage-- as long as you edit brilliantly." ~C.J. Cherryh

“No, writing 50,000 words in a month is normal. You are not crazy. This is not insane.” ~Teri Brown