I have to agree with jgags6. I am a big fan of a long game and good story in single mode. I love cod4, but this big focus on multi player mode is pissing me off. Single mode is getting shorter and shorter. It seem game developer think every gamer plays online. I won't pick up a game that is multiplayer only and seem to be a few of them out now.
Lets get back to making fairly long single mode.
Well unless you just sit there and play it non-stop for 5-6 hours then yes it won't be as enjoying. I am the type of person to just play through a couple of levels and then stop and play something else. So that 5-6 hours can seem like 3-4 weeks. Also there are three levels to play a game so if you play each level then it will take you a long time. Also there are achievements (xbox360) that you can get and that makes it even more challenging.