I actually thought Nav was one of the easiest of the exams. As Send Clowns said you'll need to plan a route but the first 50% of the questions all relate to that route, so if you've done it properly you should score 100% for the that first half. I'd already done quite a few XCs before I took the exam (including my QXC) and although daunting at first, it becomes second nature after a while.