This of course relieves the pressure to present an offline Garmin gpx export solution 
Additional comment on the above test with the track distance ticker.
The track distance tickmarks, as routewaypoints, are precisely synchronized with the track position, such does not mean are synchronized with individual trackpoints!
GPS Visualizer: Convert GPS files to plain text or GPX The distance tickmarks where generated by track import and the function: Add equal-interval tickmarks as waypoints.