Linear = no branching pathways. Not strictly underground. There are a lot of well-lit outdoor, areas through forests, cities, what have you. But it is not literally a tunnel. The first few areas of the game you see in the map are in fact a ruined elevated train track running through the core of a moon, so yeah that's going to be a relatively straight line no matter what.
That '10 hours' thing is probably the most overquoted blurb about the game. It took me less than two hours to clear all that. Anyone who takes longer than 3 hours is really, really terrible at a pretty easy JRPG.
XI Wiki


