Battery life in “GPS Only” mode

The specs say up to 42 hrs. Furthermore…

The actual battery life depends on the features enabled on your watch, such as wrist-based heart rate, smartphone notifications, GPS, internal sensors, and connected sensors.
Mode
Battery Life with Always On Display
Battery Life with Wrist Gesture

You might need to play with a few more settings. 

Yeah I have WHR and notifications turned off all the time. In Power Manager I have GPS only mode enabled and connected sensors (and everything else) turned off. I'll see if there is something I have missed tho, thx. 

According to dcrainmakers article the difference that take gps power profile from 30 to 42h is turning off always on display and using gesture mode.

I just verified that, indeed the claimed max battery on ‘extended’ profile with gps only shows up as 42h

Maybe look at the brightness level? Do you have Pulse Ox enabled??

Must admit, the battery estimates confuse me, irrespective of the brightness level, if I charge to 100%, and in Gesture Mode, the watch never displays a claim more than 10d of 'smartwatch' operation, even though I know I can easily extend beyond that, close to the 16 days documented claim.

(it's Wednesday now and I'm on 82%, having charged to 100% after my previous activity on Sunday 3 days ago - ie I've drained 6% per day = up to 16 days, but right now the Watch is saying 8D of battery, when I could probably eke out closer to 12 or 13 days from now - why doesnt the 10d estimate at 100% ever update??!)

Likewise, from 100%, and in GPS All Systems mode (with WHR, but irrespective of whether I have AOD on, or Gesture mode during an activity), the watch will display that there are 21hrs of estimated battery, compared to the official 24hr (Gesture) or 32hr estimates (AOD) - that presumably are defined around minimum screen brightness?

Thx. Yes, it appears turning off always on in the power manager is independent from turn on gestures in settings. That did the trick. Thx all for the feedback!

I can give you my settings if you like. I am getting battery estimates of 60h and you should have no trouble with a 100 mile race.  I had a course loaded as well with waypoint markers.  I had my inReach connected and iPhone connected but no HR.  With the HRMPro strap I am getting estimates of ~50h battery life. Why I chose the Epix, display is great and the battery is good enough.  This has been tested in ~15h of trail running so far.

Wow. I’d be interested in as many settings as you can provide. I feel like I have everything turned off except GPS that can be turned off and am not seeing anywhere close to those estimates. Thx!

I also have everything off in a "max battery" power profile except GPS (including OHR off), min brightness in display settings and gesture mode in display settings. I cant get power manger to give me anything over 33 hours.

Don't trust Power Manager numbers. I've done some long runs to test and then assess actual battery life from the fit file in DCR Analyzer. Here are my settings and data for 15h run

.

Just attempted to duplicate your settings, still get a max of 33 hours in power manager. Those power manager numbers of yours look similar to a Fenix 7!