In the high prairie north of Denver there are really no flat areas. Also, we rarely ride only 20 miles - a "standard" ride is more like 40. We almost always ride by ourselves. So, the rides range from rolling hills to canyon climbs & descents. Team age - about 103.
A descent average is about 18mph for about 40 "hilly" miles. A similar distance with more climbing is more like 16.5 mph avg.