Hi,
So when entering a shoe into Connect it is not possible to alter it's starting distance/mileage.
I've been searching the forum for a answer for this as well as contacting Garmin support receiving a "It's not possible"-answer,
So I think that editing a Shoes distance/mileage should be possible. For example I've tracked my shoes in Strava and when moving over to Connect most of the shoes already have a couple of Ks in the soles :)
In Connect they become brand new! For sure would be a good thing though :)