The main quest is designed to run all the way to level 50, so you have quite a lot of ground to cover. That said, the main story also unlocks a lot of unrelated content that improves gameplay considerably (inn, levequests, airship pass, duty finder/dungeons, retainers, Grand Company access/personal mounts, even certain non-dungeon areas are unlocked through storyline progression (like the Isle of Umbra).

That being said, don't feel you need to rush to the end of the story in a week - it's more worth taking your time and enjoying the story, as, apart from some minor exceptions, it is actually very good, especially towards the end.