So when I first played the game I skipped a bunch of stuff to see how long the story lasted. To my surprise it was beatable in a day or two. The second playthrough I knew what I was doing. I went as far in the story as I could go without doing any quests. When I got to the point of the game being a bit difficult i went back and did all quests and side quests I could get my hand on. When done I stayed at the 2.0 EXP INN and went from level 18 to 49.
You don't need to max out on exp as I am level 92 right now with over 1,000,000 exp and still haven't finished all quests. So yeah, eating at the right places (chocobo wiz) and making spells with exp bonus effects aren't needed. If you wish to use them do so early cause after 99 you don't need exp at all. Kind of a bummer.
The hidden dungeons are pretty cool. Once you get all the King's Swords and have finished the game you can go back to chapter 15 and complete the end game stuff. This game is fun and no walkthrough is really needed. Sure you might need to look stuff up in the end but most of the stuff it pretty much easy to find.