The app does not yet "follow" a route so it cannot give estimates to the end. It can determine whether or not you are near the route and so can give alerts when you deviate from it, but does not know how far along the route you are. This would be pretty easy for routes that do not go back on themselves, but needs some more complex logic to allow for those that do.
This is part of the navigational improvements that are joint top of my to-do list, along with the always on display. They will probably include turn-by-turn (if the GPX file contains directions), improved waypoint features and a few other things. Maybe "target workout" where it follows a previous workout route and shows how far ahead or behind you are, like with the current "target pace".
I would have started on this back in June but Apple unexpectedly announced many workout changes in watchOS 9 so I have been working hard on them since. Hopefully when thing settle down after the watchOS 9 and Ultra releases then I will be able to start on the navigation features and AOD.
In terms of the distance the app uses Apple's distances by default, so you may need to calibrate them as described here:
https://support.apple.com/en-us/HT204516. If that doesn't work (some people just don't get on with it) then you can switch the app to use the GPS distance instead in the Pace & Distance settings of the app.