Welcome back!

Well, whereabouts are you up to in the storyline? Go through the main scenario (which has been expanded) all the way to the 2.55 patch story which is required to gain access to Ishgard and the expansion's content. This is also necessary for levelling up your job past 50 as even though the new Job quests start in existing areas, they will take you into Heavensward areas and the quests are essential for gaining new abilities past 50 (your base class will no longer gain default abilities after 50 - it's only Job abilities after that unlocked by a Job quest every two levels).

So really it comes down to getting up to date with the main scenario quests - especially now as they also reward you with old 'tome' gear to help keep your gear up to date for that content.