Ok, so I'm gonna 120 my fishing tonight (I think) ! What should I do (fishing-wise) when I get there? As in, should I do some fishing quests or what?
It depends what you see your Fisher as doing afterwards; that might sound a tautology, but it does require some thought. Do you want to chase the Rare and Legendary fish? In which case, you'd possibly be better off not doing fishing quests. That sounds contradictory now! But the reason is, the chance of getting bait is HUGE from the quests, but the downtime gaining all the fish required is for the quests is colossal, and there's so many different types of bait the chances of getting the one you need for the fish you're after is very low. And the bonus from bait isn't all that great, it has to be re-applied to the rod after every cast, and you won't get many casts even after weeks of grinding the quests. You're better off just going straight to the area for the rares etc, and spending the time fishing there; save up the other fish that appear and do the quests as and when you've enough stock and inclination.
If you're out to earn money though, MiBs are still the way forward. The odds on scrolls or decent items from the fishing quests is way, way too low and demand is dropping off.
But if you're a masochist, and want to help the community, by all means do fishing quests! AND GIVE ME YOUR RESULTS! See the sticky here on the forums; so far, they're all mine... spread the fishy misery around and help out with submitting your quests please!
And in the meantime, congratulations on becoming a Legendary Fisher!