There are two pretty cool iPhone applications for runners: Fitnio and RunKeeper. It sounds like they both do the same thing, they just heave different user interfaces. Both are free ... so there is a great reason to try both of them and figure out which one suits you best:)
According to the specs, "RunKeeper uses the built-in GPS in the new 3G iPhone to enable runners, cyclists, hikers, skiers, walkers, etc. to track their outdoor fitness activities and promote good health." The app lets you track things like duration, distance, pace, speed, and path traveled on a map.
Fitnio says it "displays the same data as a treadmill or stationary bike" by integrating with the iPhone GPS technology to give real-time, accurate fitness feedback. It calculates and displays distance, speed, pace and calories burned.