I'm always blown away by the average speeds a lot of the old timers around here post (with strava charts to prove it too). A lot of those folks have been riding for years with groups and are oriented toward performance. My wife and I started riding about 12 years ago and have always been oriented towards recreation. I hit 70 this month and am more like you in the speed department. 15-16 on the flats at times, quite a bit less in rolling terrain or against winds. I suspect my average is somewhere from 12-13 but I don't bother tracking it. My wife and I ride for fun, typically in the 25 mile range. We will occasionally do longer rides but speed and distance are not what it is about for us. What makes me laugh is how impressive most of our friends think our riding is. They should read some of the posts around here - they would think we are wusses.
