leveling is trivial in this game, you just fate grind in a few zones. don't bother at all with story quests or class quests, just level as your class with level 1 gear (at 45 equip a full set of crafted/vendor gear and convert every time its 100% spiritbound for extra gil). Then at 49 go back and do your entire story and class quest. Its a dumb way to level but the most efficient, since the story/class quests has you wasting a lot of time running around all over the damn place and you don't need anything other than flash (glad x-class) and a single attack spell to gold every fate, in a group. Of course your first class you gotta story 1-20 to unlock airships and mounts, but after that its just grind fates and do quests in between fates (for gil, always choose gil). You can do all the quests you missed/skipped while waiting for fate respawns.
1-12: any of the starting zone fate grind
12 - 20: aleport
20 - 25: hawthorne hut
25 - 30: quarrymill
30 - 34: costa del sol
34 - 40: coerthas
40 - 50: n thal (esp when DD is spawning, ~1 hour respawn) and mor dhana when n than is too crowded