I think terrain has a lot to do with average speed, too. If I'm somewhere relatively flat (like on the coast here) I can average about 18-20mph. But throw in some hills, like when I ride further inland and that speed drops significantly. Many races have hills, while some are virtually flat.

It also might help if you tell us how old you are and whether you're a man or woman. Then people who are in your age group can weigh in with their experiences.