I'm totally new to GPS and tracks, but a few days ago I ran some tracks downloaded from Cannonshot. I was able to open the GPX files in the Motion-X app on my iPhone and follow the tracks no problem. I pre-loaded the map data in case I was out of cell range. The 4s has GPS capability so it would get nav data without cellular.
It worked slick and for now I have no intentions of buying a dedicated GPS unit as this iPhone setup worked great. Total investment <$50 including waterproof bar-mount case.
Only God decides the LENGH of your life, but you decide its WIDTH.