I wonder if that 20 hours number includes using fast travel or not, and if so.. which fast travel (OB-version or carriages)?
Things like that matter a lot for how long things take.
My guess is that the 20 hour mark is with fast-travel, probably the Oblivion-style. But think about what makes up those 20 hours. It's enough room for probably about 15 quests. That's an hour and 20 minutes per quest. That's how long some recent horror movies are. It's a full ten minutes longer than Land Before Time, which is a great movie that gave a fully-developed story.
So take that movie-length quest and make 15 of them. Another way of looking at it: how many plot points could there be? One tutorial quest, most likely, one quest for going to High Hrothgar, one for the final confrontation. So say about 12 more separate plot points besides those. That's a lot of room for the devs to make the journey the character makes. Morrowind had 19 quests, but 3 of them were talking to the house leaders and 4 were talking to various camps. Oblivion effectively had 17, effectively 16 since the last one consisted solely of picking up your reward. If the devs give us 15 or so (slightly short) movie-length quests that don't feel like they waste time and probably doesn't include time spent if you travel on foot, I'll be completely satisfied. Especially considering that there's that same content at least 6 times over left to be explored.
What I would like to see, which Oblivion and Morrowind both had, are optional portions of the main quest, which doubtful are included in the 20 hour estimate. In Morrowind you could take the time to kill all the Ash Vampires
if you felt like it. In Oblivion you could do the same by sending troops from every major city to Bruma, and by staying in Kvatch to raid the castle. I've got no doubt that these optional portions will return in Skyrim, and, being optional, are probably not considered in that estimate.
The Lord of the Rings' extended trilogy is 11 hours long, and includes a lot of downtime. Look at how complete and full of a story that trilogy gave. Would anyone say it feels rushed? Now think of what the devs, who have proven time and again to be very competent developers, can do with twice that amount of time.