Although road bikes are best for doing distance on roads (duh!) a century can be done on a hybrid or mountain bike. I've seen (and am always amazed) people doing it. One recommendation is make sure you change out the tires or even the wheels. I would try and get 700 V 25 or 28's on the bikes to lessen road friction. Keep in mind if riding a bike with wider tires, you are doing 3 times the work as a bike with narrow tires. In my first century I rode almost the whole way with two guys on beach cruisers. They were having a good time (and obviously being silly), I struggled on my nice road bike.