I wish that people who want help with quotations would give the whole sentence, and ideally the sentences before and after too!
This sentence probably goes on to say, "Long past the days when things were better" or something like that, and if it does, then "past" is right. It would be a poetic way of saying "The days are long past when things were better..."
But as you haven't given the whole sentence it's not possible to be certain of the correct answer.
Well if it was a long time ago then "long past"
But if you are saying that the days passed slowly then "long passed the days"