How are they going to fit the books with the show for the next 2 or 3 seasons?

So I've noticed with the filming that they plan to do scenes from the last 2 books, and I'm halfway through A Dance With Dragons right now and I haven't yet come to some of the scenes I've heard they will be filming in the show. I'm starting to worry that they are rushing this too far, and season 5 will be both books together as one, with some bits cut out, so I'm wondering how will they make use of whatever storylines they have left for season 6? And how will they continue storylines of characters that they might have finished too soon in the show? Unless they mean to pad it out with stuff that isn't in the books. I heard they plan to have seven seasons of the show too, but what if they have nothing left to go by by then? I know there's a lot of stuff that have that wasn't in the books, and the other way round, but it's not like they can completely make up storylines in case the author has something else planned in his next books, is it?