If you're going to bring your phone anyway for other reasons (camera), maybe you don't need a GPS watch but a power bank for your cell? Would be cheaper.
At the same time you can save power if you put your phone on airplane mode (and turn off wifi), lower the screen brightness, close all applications and only turn the camera and GPS on when you need it. I get 2-3 days out of my cell battery when I do that, 5 to 6 with my power bank and you can get more with a power bank that holds multiple charges.