It Counts Steps
Simple, free, basic, reliable on 6s. Easy to use - as long as your phone is in your pocket so it rocks back and forth when you walk. It counts your steps and calculates an estimate of mileage based on your height, and thus proportionally your stride length.
Like most pedometers it can be fooled by activities that resemble walking or running, but are not. Digging with a shovel, for example. If you want actual distance traveled, use a GPS based app.
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Steps - Activity Tracker, v1.4.3