In my experience a smartphone will do just fine if checking every so often (stored in a frame bag behind the steerer), or better still a tablet (on the side of pannier bag). Download an offline maps application and check it every now and then. Tablet battery life is usually excellent when only using GPS and it's much easier to read a map on a large screen, just check it every now and then when you need to be reoriented and take a break from cycling.