I started Latin in year 8 outside of school hours (but within school), taking my exam in year 10.
Not going to lie, in year 8 and 9 I did absolutely nothing. We only REALLY started doing stuff in year 10 - seriously the idea of 8/9 was learning grammar structures but I had no idea at the time what any of that meant until I did it in German.
So yes, it's very possible to do it in one year. I did the WJEC Cambridge Latin Course, Latin language + civilization. I am no natural linguist by any means ahah and the brilliant thing about it is you get a list of 400 ish words, and basically the exam is made up out of them words. Took me about 7 - 8 months spending about half hour a week on vocab to learn them all :O (almost.)
Literature I have no idea, didn't take it.
You're not underestimating its difficulty at all. It's nothing like other languages really as it's all reading and comprehensions
EDIT: Tips for getting an A - learn all vocab. Learn endings. Learn what they mean.
Seriously that is all there is to it - it's sort of hard to explain but unlike other languages for example German where words MUST go a certain place, Latin isn't like that. You can put the word pretty much anywhere in the sentence, just stick the right ending on it and it'll make sense... Generally.