If you sometimes run on a track (at your local high school, for instance), it's easy to measure your distance there. Most tracks are 400 meters (about 1/4 mile), so four laps would be about a mile.
If you continue to run outside a lot, you may decide you want to invest in a wristwatch with GPS, such as the Garmin Forerunner. You'll also be able to keep track of your pace, as well as other helpful running data.
Also see: What's the Best Surface for Running?
Safety Tips for Running Outside



