I searched for this and found a thread in "Troubleshooting", and felt it needed to be here too.
The ability to edit distance would also be great for mountain bike riding. My Edge 305 always is around 10% low on distance when riding trails. It is dead on on the road. The link below is a recent ride that my Edge measured at 16.67 miles, but my Cateye (calibrated to the Edge on the road) measured 18.57. This happens on every ride, regardless of the recording option selected (smart recording or every second recording).
https://forums.garmin.com/showthread.php?p=15150#post15150