Ellen, I wouldn't take the average speed on your trainer as indicative of anything. Depending on the resistance setting I had mine on, I tend to either average 18-20mph (way better than I can do on the road) or 12-13mph (slower than I ride). Trainers provide a great workout; I use mine often in the winter, but the average speed you record there has nothing to do with the speed you'll be able to ride outdoors due to the varying levels of resistance. So, just enjoy the workout, but I wouldn't take the speed too seriously.
Emily