Does Apple Watch automatically stop walk/run workout?
watchOS 6 (set to release in fall 2019) will include enhancements to automatic workout detection.


Apple Watch: All of the health and fitness features, explained



  Automatic workout detection
  
  When the Apple Watch senses you're beginning a workout, you'll get an alert asking if you want it to start tracking. You can respond to that notification to choose your workout type and beyond a session.
  
  Best of all, if you missed out on a few minutes before beginning to track the workout, Apple Watch will give you retroactive credit for that, too. And if you forget to stop tracking when your workout is over, the Apple Watch will detect that and automatically end it.