For a century, I really don't recommend trying to hit a target speed. As mentioned above, your speed will vary significantly over the course of the ride depending on terrain, winds, how tired you are, etc. I recommend that you find a cadence range that is comfortable for you and ride to that, no matter what gear it means you're riding in. Personally, I find that 70-80 is a good cadence I can sustain for long periods of time. Your own range may (probably will) vary from that.