I saw the game design document that said that the targeted game length was about two hours. My first game took 12 hours; Two six-hour sessions. My second game looks to be taking about the same length; an hour on a level seems to be typical.

How long do your games take? Is this just a case of "it's only my second game, and I'm being too cautious"?

No, the game can take a lot longer than 2 hours.

Note that the number of levels increased during development. Very early builds only had 3 levels. Later for a long time the game had 6 levels. It may be that the increase to 12 had the side effect of increasing game length. I now play a game of DotE over 4 days.