A 5-hour century is a respectable goal. I started riding bents with a goal of 2.5 hours for a 50 miler. That took a couple of years to attain, which I did by way of fairings. I never managed a 5 hour century on the old V-Rex, although I had some good 50-60 mile rides that were easily on-pace. The real breakthrough happened when I got my lowracer. I imagine that a highracer would also do the trick, but a 'standard' bent would require more training and improvement than I could manage. Of course, adding more hills will drop your averages no matter what you ride. I have lots of small hills, but no major climbs or steep grades.