Yardstik

Measure distances simply and accurately on your Apple Watch without starting a workout.

Yardstik features two measurement modes: Open-Ended and Set Distance:

Open-Ended simply measures the distance between two points. 
Set Distance enables you to measure a specific distance and receive audio and haptic feedback when you reach the target distance. 

When using Set Distance you can also enable Interval Mode to receive audio and haptic feedback each time that distance is reached until you end the session.

Uses include: Workout route planning, interval or recurring distance training, pacing distances, and general outdoor distance measurement activities where your GPS location data will be available and sufficiently accurate.