He even specifically says it is a monster "1500 page manuscript like the last one", so he must be pretty far along. I know there were a few hundred pages left out of the previous book, and a few chapters he had written waaay back that keep getting pushed back, so it is possible he may be well on his way to having something to turn in this winter or next year.
Remember that the last 2 books were in large part 'filler' while pieces got moved around to where they needed to be to for the lead-up to the climax. Jon, Bran and a few other characters obviously had some very important events happen to them, but it is still mostly setup for what is to come. Now that everything is primed to go, I don't think it unlikely that his enthusiasm and pace have picked up as well.