I was only about level 45 when I entered the N-Zone, and had no problems beating the game, so I wouldn't worry about leveling up so much.

However, mastering jobs is really key to beating the game. When you master a job, you transfer important abilities to your bare character which can come in very helpful as the game proceeds. Also, the higher level abilities you learn from mastering jobs, like Dance! and $Toss! and the level 6 magic spells are invaluable. My advice would be to make sure all of your characters have at least mastered a handful of jobs; preferably, every job should be mastered by at least one character.

Also, although you should be able to beat the game in the 40-level range, just remember that you can no longer gain experience points once you enter the N-Zone, so the level you are when you get there is the level you will remain at. You can still gain AP, however, so mastering jobs is still possible. Good luck.