In reply to Newbuild100:
Before I got a dedicated GPS unit (well began using my wife's old one before getting one myself) I used the Strava app on my iPhone. Even with the unit fully charged I'd only get about three hours of tracking. You can get more by setting the app recording then, somewhat counter-intuitively, turning the app "off". It doesn't actually turn off but goes in to background mode, iOS recognises that it is still active and doesn't clean up.
I'll add to what the other replies have said - get a dedicated GPS unit, that way you save the batteries on your phone in case you need to use it for an emergency call. You can plug in something like a Powermonkey to extend battery life if you so need.
I've now got a Garmin 510 for biking and I've yet to get anywhere near draining the battery even on long rides. I think it's meant to have a 16hr battery life.