Before my first 145 km (+/-3500m) or 90 mile (+/-11,500 ft) ride I'd ridden about 500 km in the past two years with one ride up around 60km, although I'd averaged 80km (50 miles) a week running for the previous year. Although it was a little shy of a century we only stopped at this distance because that was the "my house to the top of the climb and back" and I have no doubt in my mind that I could have finished 100 miles. If you have a pretty decent base level of fitness, don't push the pace early and eat and drink then I don't believe you need any real specific prep for a century.