Pascale,
You are not alone. I put about that many miles on mine going bewteen 16-21 mph. I *want* a road bike, but I never would have gotten on it. With my "comfort" bike, I am pushing it to the limits and loving biking (something I was terrified to do for 37 years). I am practicing, and gaining strength and endurance, while saving up for a "real" road bike.