There's a lady at our local Sweatshop group who can run 5k on a treadmill but found it quite hard going initially on the runs outside. I think the variation in hills is harder to replicate on a treadmill.
I also agree that you should get yourself analysed. I can't recommend my local independent store as they sadly closed down, but they were fantastic. Apparently I'm a neutral runner so hopefully my Asics and my style will reduce the impact of me pounding away on tarmac!