You can easily level 2 or even 3 classes, just going through the MSQ and doing side quests. I leveled Dancer (which is my main now and it is very good support), Bard and Red Mage in SHB through msq and side quests - I like doing all the quests, I'm a completionist I guess. If you seem to be behind on one, do a level duty and you catch up quick. Welcome back, and good luck with whichever you choose!