I asked about this before when I first got the S3 after using Garmin. Garmin shows mileage when you run on the treadmill. So I was surprised that S3 doesn't. Anyway, just input your distance manually.
Endomondo doesn't show mileage either when in treadmill mode. On my recent 6k walk outside my GPS did not lock at all for that whole hour. At the end, the result doesn't have my walking map but it did show a very close mileage when I measured the distance of my walking route in Google map.