All out only 18? Should be able to hit 20+ on flats with a decent bike.
I'm currently averaging just over 13 mph on 20 mi rides in SF, with lights and stop signs everywhere, and generally not safe to be over 20. And I just started riding after a 3 year break. I do wonder how some people get so high of an average by "cruising." Maybe if most or all their miles ridden have no stops or slowing.