Hi Jon
According to Apple, "Every full minute of movement equalling or exceeding the intensity of a brisk walk counts toward your daily Exercise and Move goals. Any activity below the intensity of a brisk walk counts only toward your daily Move goal."
It is possible that your walk, although strenuous (especially given the terrain), did not meet the watch's determination of brisk exercise. Also check that you are wearing your watch snugly during exercise, which enables the heart rate sensors to work optimally.
Further calibration over time can improve the accuracy of calorie, distance, Move, and Exercise estimations. Note that the calibration instructions advise finding "an open, flat area outside that offers good GPS reception and clear skies".
Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support
If you unpair and re-pair your Apple Watch, one of the few items of data that you do lose is calibration data (also synced playlists, cards used for Apple Pay on your watch and watch passcode) - so you could certainly do that:
Unpair your Apple Watch and iPhone - Apple Support
set up and pair your Apple Watch again