I use the MapMyWalk app to track how far I walk with my phone. It uses the phone's GPS. It's generally pretty cool. Today I changed up my route a little bit and knew I did over 3 miles but I forgot to hit the "Start Workout" button so it didn't track anything! 
I logged my walk in the app as 3 miles but I know it was more and that's gonna throw off my stats for the week. *facepalm*
I logged my walk in the app as 3 miles but I know it was more and that's gonna throw off my stats for the week. *facepalm*