One thing to keep in mind with Game of Thrones (and something to it's advantage compared to other shows) is that there is an over-arching story which has a definitive ending, which George R. R, Martin (author) and the show-runners already know.
Even though the final 2 books haven't been written yet, there's still a conclusion that the show can build towards, which means less chance of the story just petering out.
It's up to the show-runners how they pace the series to get to that ending.