Your statement about 80 miles was in answer to the question of how many miles you ride per month. If that is indeed what you meant - 80 miles per month - then that translates to just a few miles per ride. And you say that your commute at college would only be about 15 minutes. Given those parameters, it sounds like your current bike would be just fine. Yeah, a hybrid would be slightly faster than a mountain bike, but I think you'd need to be riding a lot more than what you say before you'd really notice a huge time difference. My recommendation would be to just stick with the bike you've got. And if you're set on getting something new, I would agree with much of the advice you've already received - get a cheap used bike, or at most an entry-level bike shop bike such as a base model Giant Escape, Trek FX, etc.