as the topic said...
what if like i got my 1st class/job to max level
which means i finish almost all of my side quest+MSQ
how am i going to level up my secondary class/job?
fate/dungeon/leve quest all the way?

Dungeon , Guildhest and Leve quest .

Dungeon roulette is good XP per day for each of the roulette, daily hunt will still give XP, challenge log, FATE, leve, if you are leveling one of the ARR classes then hunting log.

im planning to level my monk or AST after getting my SCH to 60. been dreading on how im gonna level them. FATES in the new area,seems so slow on killing the mobs.

Although I am new, but i know a few ways.

Hunting log
fate
leve quest
guildhest

ty all for the answer
im just glad i dont need to do anymore boring missions

The new jobs doesn't have hunting logs. So its either leves/temple leves/dungeons/FATES/guildhests.

Also don't forget that the expansion is new and currently the differences between XP gain and the XP needed is too high, I am sure it will be adjusted later on.

So for the moment leveling alt class will be a pain from 50 onwards. I still have left over quests in some of the new areas that I can do, being able to fly makes them a breeze to do anyway so I can use those for alt class.

find a slave tank/ Healer if you are trying to lvl a DPS in dungeon. Tank/Healer always has the fastest Que. or Fate, they are the 2nd most fastest way for DPS class to lvl up fast.