Fitness app

See the last paragraph in the 'Get reminders to end your workout' section of:

Use the Workout app on your Apple Watch - Apple Support

"...open the Settings app on your Apple Watch, tap Workout, then go to End Workout Reminder."

Go to the watch app on the phone and tap on workout,turn off running autopause and end workout reminder.

Or on the watch in settings/workout and turn them off there.

Or;

When you start your workout slide right on the screen and tap on lock,then when you finish the workout turn the digital crown in one continuous movement to turn the lock off then slide right again and tap on end.