What sort of instructor is putting somebody up for the nav exam without doing any navigation with them first? (Well, possibly one trying to bring somebody down to size I suppose?)
Cancel your exam, plenty of time to do it once you have some useful experience to built it on. Also get some proper groundschool, this really isn't a subject to self-teach.
Possibly see if you can back seat with somebody else doing nav training, or pax time with an experienced PPL doing a $100 burger run.
G