As others say, it depends. For me, a reasonable cruising speed on the flats, solo, for several miles, is about 18-19 MPH. (recovery/slow is 16 and pushing it is 21-22)

If you want a better, more objective measure of your effort, get a heart rate monitor and read up on the "training zones"....or if you have lots of money you could even get a power meter.

Most important...Enjoy the ride.