Fitness App incorrectly measures Half Marathon

If you are using the default Apple Workout App, did you try to use a 3rd party app, for example WorkOutDoors, to compare the results?

In case only the Apple Workout App shows this error, you should let Apple know about the issue by using the Feedback link. This is the only way to reach them directly.

Feedback - Apple Watch - Apple

Which Apple Watch do you use?

Did you already calibrate your watch?

Calibrate your Apple Watch for improved Workout and Activity accuracy - Apple Support