If you need competition of sorts in order to get motivated, then you'll love the new TAG feature in the Nike+ GPS iPhone app. It basically lets you challenge friends based on distance, time, or who's the last to run.
The way the feature works is that you're prompted to invite friends to play tag after completing a run. You can invite as many people as you want, track the results, and even trash talk. The only goal—as with the traditional game of tag—is to avoid being IT.