I suppose it is possible to finish the game in 20 hours on your first play-through, but you probably missed quite a bit.
Larian's lead QA can finish in 10 hours, after playing the game for 2 years and memorizing everything (
This game is short).
What level did you finish the game on? Were you playing with a no-DVD crack and got 3000 skill points at level 15, or something?
You can continue the story in
the add-on (to be released around September), either from a previous save or with a new character.
