how hard IS a century?

[Starfish]

Long unsupported rides do call for a Camelbak unless there are convenience stores along the way.

One other option, if your bike as the mounts for it, is to carry a 3rd bottle. I do that on longer unsupported rides (on the bottom of the down tube). There are also those bottle mounts that go on the back of the seat...I've never had one of those.

I have a camelbak, and used it when I first started riding, but I find I dislike having my back surface all covered up.