There's no reason they couldn't have come up with more story material to properly fill twenty hours of television. All the padding and delay to the serial narrative was simply lazy writing.
Coming up with story material is one thing. Obviously, they came up with 20 hours' worth of story.
The hard part is turning those ideas into scripts. I've seen a pretty clear pattern where shows tend to be a lot better when they have short seasons (in the 13 episode range), and tend to take a drop in quality if they have to fill out ~20 episodes instead.
The most acclaimed shows on TV these days tend to have short seasons. I don't think that is a coincidence.