David Smith wrote about using his Apple Watch as an 'adventure watch' similar to a Garmin or Coros Apex Pro. What he discovered is that if you use the Apple Watch with the display off (via Theatre Mode), you could significantly increase the battery life (up to 3 days).
It is becoming clear to me that Apple has chosen to spend their battery budget on illuminating a power hungry Always-On display. Delightfully, an unpublicized corollary to this is that if you turn off the display, the possible battery life is now massive.
There have been some rumours around a durable 'Casio G-Shock' style Apple Watch circulating, and I wonder if it will feature a mode like this for extended backcountry camping/hiking trips.