Living in the hills of the SF Bay Area, I don't think of rides so much by horizontal distance as by how much elevation gain they have. I don't think I've ever done a century with less than 5,000' of climbing and 8'000' is about average. Doing that kind of climbing in 5 hours plus riding 100 miles just ain't going to happen. It's difficult for me to imagine what it's like riding 100 miles over flat terrain.