Note that if you want to use GPS but do not want to incur roaming charges, turn off your *data* but do not set your phone into Airplane Mode. Airplane Mode turns off *all* antennas. You want to leave your GPS antenna enabled -- you do not incur any charges for data when using GPS.
Note that in some apps, having data enabled *does* apparently improve accuracy (I say 'apparently' because I haven't verified this myself). With data enabled, your phone is able to triangulate by the cell towers as well as the GPS satellites. However, for something well-marked like the Camino, the improved accuracy isn't really necessary.
One other thing to note: Run Keeper and other GPS-enabled apps do tend to suck up a lot of battery if they are used constantly. You may want to do some pre-departure testing to see if Run Keeper kills your battery in typical Camino conditions (ie, 20 - 30 km / day). When I use GPS-enabled apps on long-distance hikes, I typically only pull my phone out to check my location or look something up; I don't normally leave it on.
Cheers,