Mancare, needs to be dealt with like "piacere." Just a small group of verbs that you have to think of in this way. Check out http://italian.about.com/od/verbs/a/italian-verb-piacere.htm
Yes this does not even make sense. why do we need to talk about to be finished. You've already said it. But still it looks the same as English. I would only 80 kilometres to go means the same as as 80 kilometres are missing. And none of that relates to how many kilometres have already been covered.
Are you really expecting those who construct duolingo to demonstrate breadth of vocabulary or even depth of knowledge of grammar? I have worked 4 languages on this site and found each very sadly lacking in depth of knowledge of synonyms in all languages. Even the first definition which appears for a foreign word in a dictionary is often rejected and the solution given not even in the dictionary. Correct English grammar answers are occasionally rejected and incorrect grammar given as the correct response.
It is, but I believe we do not know at all what the subject of the sentence is. The verb is in third person plural because the 80 kilometers are missing. I translate literally to 'are missed, 80 kms', but we do not know by whom. They could be missed by me .. 'Mi mancano 80 kms' or by us, 'Ci mancano 80 kms.', or as you suggest, by them, 'Gli mancano 80 kms.'
Just to correct one thing: we actually do know the subject -who? - that is being missed: them 80 kilometers. It is the object - by whom? - which is not indicated. Had it been, it would have appeared at the beginning of this sentence because of the 'piacere' pattern of 'mancare'.
'remain' is the third person plural of the verb, qualifying 'kilometres'. It appears that Duo is using the verb 'mancare' (to miss, to lack) in favour of 'rimanere' (to remain). 'mancare' has the construction 'subject, indirect object, verb: literally 'something is lacking to someone'. I hope that this helps.
[If French is available to you, 'manquer' (to miss, to lack) works in the same way.
For example, 'il me manque' (I miss him) - literally 'he is lacking to me'.]