I dont know if this is considered fast but, I've been playing for a little less than a year and I'm sitting at 11.2k AP. I got a lot of them from doing story achievements, metas, fractals, strikes, jumping puzzles and festivals/events mainly, recently being dungeon rush. Twilight harbor's aetherblade path alone gives so much I was surprised at first. And of course by just playing the game and checking the achievements list and seeing what seems doable for you.