So, in preparation for pt2 I finally sat down and played through pt1. I can see and agree that on easy difficulty if one does not persue any side content it would last about an hour. I took my time, listened to conversations, looted stuff, got most of pt1's related achievements and took almost 10 hours to do so.
That said, the ending to pt1 is epic. Totally did not see that coming. I'm not going to spoil it here but I will toss up a eurogamer url to an interview with the lead developer of the series:
Explains the ending, which also sheds more light on infinite's original ending and whatnot.
-cant wait for pt2 now!-