Yeah, with the newer 172's I can understand that. But the older ones, they are pretty much the same I think. I would say you can jump from a 152 to 172N/P, and have no problems flying it around.
Again, an hour or so is always a good idea, but 5 hours 152->172 sounds like alot. Unless its a G1000 transition at the same time, or something like that.