I ride with my wife every weekend. She rarely likes to go over 30 miles. When I ride with her, we piddle around 12 to 14 mph avg for the ride. She takes breaks as well...usually a couple during each ride... we set on a bench and have a drink, eat a cliff bar and enjoy the view for a few minutes...Our goal is to get a bit of exercise and enjoy each others company. If I pushed her, I would be riding by myself.

When you read about folks on this forum, you are in a bicycling world, everyone rides, many are avid riders, many do endurance. Do not think of your wife in those terms, and do not let her think of herself in those terms, comparison is not good for the ego.
Think of what she is doing in other terms, unless you live in a super biking community, how many other women her age are out there riding 20 miles on a bicycle. Let her know how well she is doing and how much you enjoy riding with her. If you want her to do better, tell her she is already doing good.