The main story is rather short, and frankly, the amount of effort that went into the extra sections of the game would gave it a sense of "you could possibly play this forever." And frankly, I enjoy a game that I can keep on coming back to despite beating the ending and getting max level.
Really, if you want to follow the main story, look for the quest at the top of your quest list. That should be the story quest. Every other one is optional. Look at either gamefaqs or speeddemosarchive to see how quick the real storyline is.
(SDA has a guy who beat the game in 30 minutes. He glitched a couple of the main quests, but even still. JUST doing main quests might level you a few levels, but should be done in about 3 ish hours.)