It also likely depends on how fast you push it. Do you stop to read books to understand the backstory of everything that's happening? Do you talk to people? Do you adventure to find better gear and get gold to buy better gear, or do go with what you find on the way? Do you take any detours at all?
That's also true. In reality, it will vary depending on how you play the game. Not just the things you mentioned, but also things like whether you use fast travel or not, and how often you use it when you can (Assuming the 20 is accounting for the time spent traveling between locations.) what kind of character you're playing and a lot of other questions. Since we're not told about details like this, it doesn't tell us much, and because of this, I don't usually put much stock in this sort of estimate beyond using it as a general guideline by which to compare the length of games. For example, if Bethesda gave an estimate that a certain other game's main quest was thirty hours long, than I'd assume its main quest is longer than Skyrim's, though for that to be reliable, the estimate needs to come from the same source.
For me, I expect the main quest will take a lot longer than 20 hours. In part because I don't generally start the main quest, then go through it as quickly as possible without doing anything else. In the Elder Scrolls, one I've started the main quest, I'll generally divide my time between it and faction quests, side quests, exploration and other things, so even if the estimated length of the main quest was half of what Bethesda has said it was, I don't think it would feel too short, unless the flow of the story suffered due to its length or lack there of.
Didn't Pete say that the radiant story did not mean random quest generation on twitter? Something along the lines of 'If you think Radiant Story means randomly generated quests you misunderstood what it is.'. From what I've understood there are quests which can select a random dungeon that you haven't been in yet as a destination for it, but not that the quests themselves are randomly generated in the sense that it generates new quests over and over.
It's been said that it's not randomly generated quests, yes, but it still seems a bit confusing as to what it is if it's not, because I've heard that it can do what you described, and that sounds like pretty much what randomly generated quests usually entail to me.
Whether you call it randomly generated quests or not, though, the concern in this thread is still valid, I think, because either way, we've still been told that Radiant Story quests can randomly select a dungeon you haven't explored to send you to, and that doesn't answer how getting a new quest will work if there's no more dungeons you haven't visited to select from.