I wouldn't stress too much about the tires, honestly -- I used to ride all over the place on my hybrid, and while I have a road bike now, I've done 70-80 miles on the hybrid quite a few times. While good tires are really nice, the biggest issue I ran into was lack of hand positions -- my hands would start to go numb. If I were going to do a century on it now, I'd get some bar-ends to have another way to grab on. And if you don't have gloves and shorts that fit well, look at those.