your fault if you don't rp at all. :-/
This is a problem for many
Entire perk trees for both Vampire and Werewolf
10-20 Hours of main story. Meaning it's still not counting the crossbows, Vampire Lords, Side Quests (you know there's at least going to be one. It's a Bethesda game), perks, the new shout, mounts, etc. That's at least 30 hours total, including main story, if you want to max out the perk tree, at least that's my guess.
You forget that you will also be able to make crossbow bolts
I do RP. Just read the DID threads. My point was that Bethesda love to make out that their products have many more hours to the average gamer than they do.
I dont fast travel, but not doing so just to make questlines longer is just an artificial method of lengthening the experience.
Not doing the main quests in each questline one after another also helps. Break it up with some radiant quests or go do some random quests.