indoor walking distances

To improve the accuracy of pace and distance for Indoor Walk, first accumulate at least 20 minutes of outdoor walking using the Workout app to calibrate your watch. For Apple Watch Series 1 or earlier, you need to bring your iPhone along for calibration. For more info, see:

Workout types on Apple Watch - Apple Support

This article will explain how to calibrate your Apple Watch to get more accurate measurements: Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support.