I was messing with my watch 3/4 of my way round a long walking route and discarded the previous 4 hours of walking. Is there any way to add the route taken to an activity post-completion?
Maybe. Is there a chance you used LiveTrack and can go to the link for the discarded activity? If so, you could use this site - LiveTrack export, to convert it to a .fit file that can be uploaded to Garmin Connect.
If you didn't use LiveTrack, there is a slim chance you can recover, at least partially, the discarded activity (.fit file). See related thread - how do I get my activity back if I delete it?
Actually, it's saying this when I put the address in :