I found a really cool iPhone app today. It will log your run/walk/cycling workouts using GPS technology.
It tells you how far you walked, how fast, it maps your route, and records all of your workouts and saves them. After a week or a month it will give you stats as to how many miles you logged.
Just go to the app store, type in "map walk" and a list of apps will pop up. This one is by Sprint. It cost $3.99 and is well worth it.
Update: September 19, 2010. The app is great but I've noticed when it's cloudy out the GPS has difficulty getting a lock. So I walked for an hour and didn't get any credit. Well---that's not true. My doctor has me wearing a pedometer so my step count was accurate on that. Maybe just go with the free app like i map my run.