Average speed is bogus unless you are comparing the same route with similar conditions. Your average speed will also go up or down depending on if you are riding solo or in a group. On a completely flat course with little wind (bike path), I can easily average 21-23 for an hour or so. If I'm riding with others taking turns pulling, I can average the same speed for a longer duration, or crank out a higher speed for the same duration.
Plus hills can put a big damper on your average speed. On our moderately hilly rides, we will average close to 18. But on really hilly rides, we'll only average 15-16.
Go out and ride with other stronger riders. Even if you just sit back and never do work, you'll still get a feel for your level and get stronger. If you decide to pull, don't feel obligated to do long hard pulls (newbie mistake), burn out, and get dropped when you pull off.