Was halfway a 15 km run so it's frusterating this (to my knowledge) is not an option.
Found others complaining without getting an answere so I already submitted the idea via https://www.garmin.com/en-US/forms/ideas.
If anyone already knows if there is a possibliity to do this, let me know!
(During the run I was hoping there were online tools to correct this but coudn't find any).
So I now have the strange case that my Garmin Coach is telling me "There is room for improvement" while I complete the workout well over the requested distance and under the requested pace (sigh).