"Is it far away?"
Translation:Er det langt væk?
No, I don't think that is right. From the lesson notes, many adverbs are created from a related word with a "t" added, but they do not decline. If "lang" means "long", then "langt" meaning "far" is a related word, but it would still be "langt" even if the subject of the sentence was "den".
Yes, that would be correct. I'm not sure what was going through my head at the time
somewhere else (I forget where) someone explained that langt is distance, and lang is time.
"Længe" is the adverb referring to the length of time but "lang tid" (or any other non-specific period of time) can also be used, but here it's being used as an adjective so has to change according to what it's describing..