How old are you? What's your general fitness like?

I'm 42 years old and in marginally good shape most of the time. I commute 14-20 miles round trip each day. In a pinch I could probably do a metric century without any other preparation, but I generally try to get in 20, 30, and 40 miles on a single weekend ride in the weeks leading up to it. Being prepared makes it more enjoyable.

For a regular century, I definitely need to prepare. The level of climbing involved is also worth considering. Many organized centuries have a lot of climbing. If your training miles are on relatively flat ground, the mountains can crush you. I speak from experience.