Everyone has a speed that makes them feel good about themselves. It is usually a little faster than the speed you can ride all day, since you then feel you have accomplished something. Or it is a little faster than the last time you rode the same route. The important component here, is that you are measuring yourself against yourself.

And this speed will change with time. It will go up with increased fitness, then eventually down with age. Enjoy your ride, and ride often, at whatever speed you choose. Riding for competition is an entirely different thing.

That being said, I must admit that I feel some dissappointment when someone way older than I am zips past me whistling a tune while I am grunting and gasping along at near maximum effort. At these times I fantasize about that new, lighter, sleeker, faster bike that will make up for my shortcomings and reverse the roles next time we meet! LOL.