I was always a 16-17 mph rider on my uprights. It took me several years after getting my first recumbent before I finally broke the 2:30 mark for a 50-mile ride, and a few years more to break 5 hours for an imperial century. My best overall was a one-way gran fondo in which I did 107 miles in 4:30. My best one where the begin and end points were the same was 4:40 for 100 miles. I don't do as many centuries anymore, and when I do I tend to socialize at the stops; so I haven't done a century for time in about 5 years.