Last summer when I started riding I did 10 k each way to work. After doing that four times I went and rode 60 k one morning and felt absolutely fine afterwards. If you're already in good shape I think you can do the century.

Is the century supported? Having food and water along the way without having to carry it should help. The local MS Tour here is 90 k each day for two days and they recommend that people ride 30 to 50 k twice a week and enjoy it to get ready. I think they "enjoy it" part is key. If you struggle to do 50 then you'll have trouble doing more, but if you find your training rides easy then you're probably ready for more.

I'm one of the least experienced riders here, so just take that as encouragement but not advice. I'm interested in hearing what the pros say.