I think I'll wait and see some other reviews on Ground Zeroes before I believe this two hour report. I mean, I could spend two hours on the first stage of MGS4 if I wanted to dick around (which I have done), or I could finish it in 10-15 minutes if I just blitzed through. Also, I think GZ and PP are supposed to be mission based, so there's likely side missions in GZ that GameInformer didn't have time to play.
I haven't finished it, but Peace Walker had the main storyline missions and then a bunch of side missions. If you just did the story missions, you'd miss out on a lot of gameplay. I think the same thing is happening here. I believe GZ is going to be more like 10-15 hours of content, more if you try for a perfect run.
I'll wait until March and see, but I'm probably going to pick this up as the demos I've seen look amazing.