The Apple Watch Series 6 is rated for 7 hours of battery usage while recording an activity with GPS. That's right at the edge of some hikes I've done, and it would leave a multi-day hike out of the question.
Compare to Garmin: The Fenix 6 is rated for 36 hours of GPS activity recording per charge, or 72 hours in a battery-saving GPS mode.
It's the 5th-gen watch, it has never lasted two days (including sleep with the display turned off) even when I don't use it for navigation or workout tracking. It also charges much more slowly than the 3rd-gen that I had before.
Is it cellular? I hear that can kill battery when not near a phone or wifi--especially when hiking where cell service is spare phones tend to use a lot of power trying to get a signal.
It might be possible your battery has degraded considerably?