Some people - I think between 10% and 20% of the population - are non-responders to aerobic exercise. They can train and train, but their VO2 max and aerobic threshold remains the same. You may be one of those.

The good news is that the exercise is still beneficial from the point of view of health indicators like BP, weight, insulin resistance and all that stuff. And the fact that your athletic prowess doesn't improve is a small matter compared to that, and to the fact that you're having fun.

And if it's any consolation, when touring I frequently average 10-12 mph. And I have just as much fun as when burning it up on my road bikes.