Don't worry about asking "average speed" since it varies greatly from person to person and ride to ride. Just establish a baseline, and then you'll have something to compare your own personal growth. I just started riding this summer, when i started, I could sustain a 15-16mph average on flat, today I was sustaining 20mph on the flat.

With cycling, you'll discover it's easy to make big jumps. If you're doing 15miles now, before you know it, you'll be doing 25, then 50, then 100(km) then 100 (miles).