Strava

What does the distance show on the device ? Corrupted file or corrupted transfer ?

If corrupted transfer (i.e. ok on device), you can try deleting from Strava and re-uploading, pulling the file directly from the device if needed rather than going via Connect.
If file on device wrong, you can try editing it in something like Golden Cheetah or Training Peaks. Strava doesn't allow you to do much (if anything) with the data itself

Pretty decent average speed...

Strava uses the GPS data as last resort for the distance measurement. If you've got info from something equivalent to a GSC-10 (speed/cadence sensor) then it'll use that, even if it's woefully inaccurate or only partial data.

Stripping that stuff out of the file and reuploading will work; so that Strava will then default to the GPS trackpoints.

If it's a .fit file then convert it to .gpx (or .tcx). Then edit that and remove the DistanceMeters tags (there will be hundreds so it'll need a text editor that can do regex search/replace).

Feel free to email whatever you've got to me (alex @ <username> .org) and I can do it.