GPS'es require batteries, and keeping the GPS on all day and tracking will go right through the battery on a cell phone. If you're going to use a GPS, you'll need a system to keep it charged constantly.

Cell phones (and tablets and e-readers) aren't designed to be out in the elements for 6+ hours a day. A dedicated weather-sealed GPS is a better way to go, especially given how many cycling-specific ones are available.

You're almost certainly going to need a paper map anyway as a backup.