Like Gidonoidon said, it's a good idea to level several classes at least a little bit, rather than just taking one class straight up to level 50. (Of course, the way you level your classes is entirely up to you.)

If you want to level quickly, be sure to do the quest so you can get a room at the Inn. And then enter your Hotel Room before logging out. That way, you'll get "rested XP," which gives you a large bonus to the XP you earn. If you're casual enough, you'll be able to level entirely using the rested XP bonus.