Well this is a pleasant surprise. On Insanity the exp points must be enormous. Finished Noveria, did a couple of leftover side quests, and near the end of Vermire my first Shepard made it to level 60 and the achievement. So it is possible to hit level 60 in just two run throughs.
I may very well be the oldest member of the site, but I am far from the dumbest :)