I know exactly how you feel. I just finished my first big race (15K) and now feel like I have nothing to motivate me; no goal to set. Best advice I can give you, is try to look for a local run. It seems that most areas have at least one local run a month, and then you have a goal to meet. Plus, you start seeing the same people, which makes it more social than just training.
Good Luck... Keep It Up!
