The library got this in, so I'm going to pick it up tomorrow or Friday. I'm busy with other games, but there are a lot of holds on it.
Is it really only about six hours long?
Definitely not. Sure, if you rush through it without taking regard of collectibles then you can probably manage it in 6 hours. But you'll miss the core of the game if you do; it's very much about enjoying exploring these beautiful levels and finding everything. I've spent 23 hours with the game, with still a fair amount of stuff left before 100% completion.
You don't have to go for 100% to enjoy the game, but - with several exceptions, mind - the levels are generally not designed to be rushed through and are not overly difficult to complete per se, so it's a big plus if you are willing to do some collecting. That's where the difficulty often comes in, and it feels very rewarding when you collect all the flowers etc.