Seriously, try to make an outline of each storyline for seasons 4, 5 and 6, and tell me if you think it works. Tell me what Arya, Bran, Sansa, Brienne, Sam, Victarion, Theon and Dany are going to do for example.
There isn't really a good way to go about it with many of those story lines, no matter what scenario they choose when they start getting into the territory of the latter books. With nearly all of those characters, it will become a necessity to both show them less than they had been previously (which, with all of the new characters that will be introduced, was likely going to happen anyway), and giving those characters additional material (either from the book series, where certain events that have already occurred are referenced but not shown; or entirely new material to supplement their existing story lines).
I don't think D&D are foolish enough to rush through the story lines for each of these characters simply to provide material for them during the course of the season. It throws the timeline of the story out of whack, and makes the adaptation process exponentially more difficult than it already is. And in either case, the end result is that D&D have to come up with additional material. That being the case, it makes sense to pad the existing story lines out rather than extending too far past the point where their stories end in the books.
It will certainly be difficult, but not impossible for them to do. Either way, we'll see how it all works itself out eventually, and I think I might personally be more excited to see how (if, even) they handle AFfC & ADwD than I am to see how ASoS is adapted (although it is, predictably, my favorite book in the series... Followed by AFfC...). I tend to be more of an optimist, so I'm confident that if D&D have any plans to continue with the series after these next two seasons, they'll be up for the challenge of adapting the two latest books.