Is it possible to trim a workout from an Apple Watch?
As of watchOS 4.0 and iOS 11.0 there is no way to adjust the length of a workout with Apple Watch or iPhone.

I experienced the same issue in the past and the only known way to do this is by


creating an encrypted backup with iTunes and to
edit the Health.app SQL database's entries
Restore the backup to your iPhone.


You should let Apple know that this is a feature you want to see in future releases:

https://www.apple.com/feedback/watch.html