I have the training and know-how to rely on map/compass, but it sure is a lot less mental-figurin’ to use my phone, InReach and backup external power. Call it lazy, call it an efficient use of my time and energy in the field, but it works for me.
Two years ago I did a 10 day hunt in Ak with this setup and it worked flawlessly (amid dense fog where traditional map/compass would have grounded me. It’s easy to say that you can navigate in the dark (or dense fog), but confidence wanes when you spotted a buck a mile away across broken terrain full of cliffs and incredibly dense undergrowth..... can you really pop out at 300 yards using a map/compass without risking injury or blowing the stalk? You could, but I can do it a LOT faster if I chart a course through that maze on my phone and follow the bread crumbs. I can also get a really good idea of how far away I’ll be when I reach my destination. What I really like about InReach is the mapping capabilities PRIOR to the hunt, so many options (USGS, satellite, etc). You can plot routes and waypoints, determine distance and elevation, and everything syncs up to your phone, so I have high-def maps with preloaded data ready to go. If my phone fails, I still have the standalone capabilities of the InReach, with all screen, navigation and text/PLB functions.
As long as your iPhone is less than 2 years old (when they miraculously loose battery life capabilities, overnight), you should be able to get 2-1/2 days per charge if you keep it in an inside pocket and warm, on airplane mode. The GPS still works on airplane mode and it’s still bluetoothed to your GPS. The InReach can work in harmony with your phone or it can work independently. It’s more fluid to use your phone because the screen is bigger (but you are burning battery for both). I would sparingly use my InReach (because that’s the device with PLB and will get you out of the worst-case scenario). The InReach will last for about 5 days if I’m not using the track function (so much longer if I use it sparingly and only turn it on to send messages to my wife at night and for occasional stalks or wayfinding).
The external battery pack I have will give me about 6 charges for either device. If I enter the field with full batteries then I can get through a 10 day hunt just perfectly.