Great question — answering as the UphillTech team:
I'm not specifically familiar with Alltrails, but what should work for any GNSS tracks website (Alltrails, Wikiloc, Komoot, others): Download the track in any common format for GNSS tracks, such as GPX. Open the track in your favourite desktop application for viewing and editing GNSS tracks. I use viking . Cut off the parts you don't want to have. Export/save the track to a file. Upload that file to the tracking website. Personally, I use this approach for any track I start at home as to not reveal where I live, before I upload it to a public website such as Alltrails, Wikiloc, or Komoot. I also use it for the oopsie you mention.
Full answer by gerrit: https://outdoors.stackexchange.com/a/30279 (CC BY-SA 4.0).
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