When I was younger I just cycled for pleasure so mph didn't bother me.Riding in a group when I was a youngster anything up to a 19mph average, though the most I could manage at that pace was about an hour, on my own I used to manage 16-17mph, the memory of that is what makes my current lack of pace annoying, what makes it even more annoying is mates who have never slowed down, most of them are in their 60's and 70's now, some are older, and they are as quick as they've always been.
Its only in recent years that I got back into cycling and got myself a basic garmin. I would expect to do a 20 miler at 12, maybe 13 mph. Still very much for pleasure.