I have almost as many moons as you and I am only finishing up the 4th Kingdom. Sorry to break it to you, but it seems that you blew through the story quicker than you would like to think.
As for difficulty, I find it no less difficult than Mario 64 was, maybe a little more. This is of course under the pretense that you are actually trying to get all of the moons as you would stars in 64. As for the story, all 3D Mario games can be completed around 10 to 15 hours if you focus mainly on that. Like I said, your moon count goes heavily against your claim that you gathered everything as you played through the main story.








