I rode several centuries on a hybrid, and was fine (not super fast, but felt good at the end). Like you, I started cycling to have something to do when the weather was nice, and before I new it (all in the course of 3 seasons), I was training for centuries, then buying a road bike, then another road bike, training through the winter indoors, and now racing almost every weekend! Good luck, and most importantly, have fun!

Clipless pedals. I did several centuries without them, both on my hybrid and then on my first road bike, so I am sure that you could to. However, now, I feel all uncoordinated and clumsy without them, and can't imagine riding those distances not clipped in, so like someone said above, it is a personal decision, but I recommend clipless pedals.