Sometimes Apple Watch prompts to record a workout automatically. While this feature can be helpful, if the user forgets to end the workout it leads to misleading data saved to the Fitness app. Other times a workout might be extended after recording has stopped, which leads to the question of how to manually edit a workout entry.
Don't look to the Fitness app for editing workout length. Workouts can only be viewed in the Fitness app. The only way to have control over workout data is through the Health app on iPhone.