I think you're right to be concerned. I coasted through secondary school with a 4.0..and when I reached undergrad I didn't have the tools to apply myself. I wasn't a big fish in a small pond anymore, I was near the bottom of the heap. first year I did ok, but by the end of the second I was irrecoverably lost.

my advice is to figure out what you want to get out of it. sure, maybe you need a degree, but that's a really big space. most of it seems like a pointless sysiphysian exercise. but there are some really great teachers and peers in there, and a lifelong passion if you can find it.