15mph (what you done approximately) is the benchmark my old club used to use; you'll need to average it over several hours but that'll be easier in a group, go along and give one a try. A word of caution you'll soon be bitten by the upgrade/ road bike bug, it happened to me
First welcome to cycle chat,you've started in the right direction,average is good but its all down to hills that will effect your averages as well,a lot of clubs will have two groups traveling at different speeds,have a word with your local club,but over a hour you are doing well,a road bike will be night and day on your times,but its down to you what you want to get ,distance ,speed,comfort and cost and quality,does the bike fit ,big choices,so don't jump in to fast,take advice,a club gives you companionship and encouragement so it may be the way to go,
Thank you you both for the help. I do want to get a Road Bike but saving as I found out £500 don't get much so I'll just save and wait.
I would like to get in to road racing, maybe next year or even later 2012 :-)