Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
We may need to do two steps. But lets start with this one.
o perform a master reset:
The device should now be reset to default factory settings, and the device should function as expected.
Once the device has been master reset, It should be calculating distances accurately again.. if not, then we will need to connect it to a computer and update its internal software using garmin express. If you get stuck or run into any issues trying to reset or update the watch, let me know.
I strive to provide great service, and greatly appreciate a 5 star rating clicked and submitted when you are satisfied.. I always answer follow up questions.Dustin S.Senior Technician and Network Switching Systems Operator / Maintainer.