It's in Norwegian, but the first page shows it nicely.
'Om tusen år' is an adverbial since it tells when an action is happening.
It could be 'Vi er borte om tusen år', but it's first to emphasize the 'tusen år'
As far as I know... the preposition 'in' in English referring to the time (IN a thousand years) translates as 'om' in Norwegian. But when you use 'in' for location (IN the house), it translates as 'i' (i huset)