Intervals. Hate to say it, but intervals. Your body undergoes "adaption" unless you continue to challenge it. So you may be strong, but you are (from what you describe) only strong if you are pedaling at the one pace. And, I would think, if you had to ramp it up to 17 mph average, you would not be able to sustain your turtlelike endurance (I think turtles aren't particularly endurance athletes anyhow).
Also, look at how you are riding....are you riding high cadence/low gear? low cadence/high gear? or high cadence high gear?
I am in no way lightening like fast. I average 17-18 on moderate hills, but I can sustain that pace and throw in some sprints for a mile or two. On flat I ride 22-23 mph unless there is a headwind....ugh. And I have to continually trick my body into getting fitter and faster by challenging it (if I have been standing to climb I will sit, or stand if I have been sitting, I will climb in a harder gear or make myself not let my cadence drop or speed drop below X on a hard climb).
I also ride my MTB at least once a week. It makes me stronger on my road bike and helps in my constant battle to "trick" my body.