Running dungeons for leveling XP is not time efficient. I remember WoW could be leveled by running dungeons back-to-back.

Here, leveling is best done through the MSQ and their relevant dungeon instances, and alt jobs through the daily roulette when you have a bonus, and other things (Beast Tribes and Hunts, for example.).

You also need to be aware of rested XP. You log off and start gaining rested XP. It doubles XP until you run out of rested XP (that blue/yellow sub-bar you see on your XP bar). The longer you're away from the game, the larger that bar grows.