I took advantage of two recent trips to test extending my iPhone 5 battery life to power the GPS function for all-day hikes.
One trip was a remote camping and hiking trip into Southeast Utah. The other was a more civilized series of hikes in the Canadian Rockies. Some hikes were longer than the ~3 hours my iPhone battery will last running the Motion X GPS software, even with wireless and data turned off to reduce the power drain of futile signal seeking in the backcountry.
I kept my phone charged on the trail with a small external battery that connects via USB. The Anker Astro Mini is a 2.7 oz unit that is speced to recharge my phone ~1.5 times. I never tested the limit, but I was happy with the overall results.
There are two ways to use the Anker unit. One is to keep it attached to the iPhone at all times with the iPhone cable. This results is an always-100%-charged phone, but can be awkward to use. The other is to remember to recharge the phone during lunch break.
On both trips, I had access to recharging power at the end of each day, so the ~7 hr (phone+battery) limit was not a problem.
One option for overnight trips is to carry a heavier battery. I experimented in Utah with charging the Astro Mini back at base basecamp with the 11.2 oz Anker Astro E5, specced to hold ~7 iPhone charges. Charging the smaller Anker unit from the larger one appeared to work fine.
Another option would be to figure out if either unit could be solar-charged. Or maybe charged by a camp stove.
For serious backcountry use, I would still recommend putting down several hundred dollars for a good backcountry GPS. They typically deliver ~16 hours of power on a couple of standard lithium AA batteries and the replacement batteries are light to pack.