Encrypt/mask GPS data stored on watch

Unless we saved the key on the watch ... but I'm a little out of ideas.

What about storing the key only in RAM and only when connected to the phone?
So the phone stores and provides the key. Shortly after they get separated the key is deleted on the watch. As soon as they are close by again the phone provides the key to the watch again.
This assumes that you only need to watch=decode the data while you have your phone with you. Going to a run and storing=encoding data could be done without phone still using asymmetric keys.

That should be safe as long as the phone is not stolen together with the watch .