I saw in the Facebook group "Garmin Badge Group" that you solved it by leaving and joining the challenge again.
I had a similar issue a couple of days ago with the biking challenges. The distance went down to 0 for all of them. I didn't test to leave and join. What I did was that I made an edit to each of my bike activities that I've done since I saw the correct distance. It was a bike ride the day before that caused the distance to go down to 0. After the edit the distances in all bike challenges were back to the correct value.
