For anyone who is struggling or did not get the "Back Track" trophy:
1) Go into your save data
2) Delete all your save data
3) Start a NEW GAME
4) Run (as much as the game will allow with R2) from the starting area all the way to the end (without enter acting with anything other than opening gates). Run past all proximity events, run past all motion events, run past radios, phones etc...
5) Once you've followed the main road and reached the last area, start back tracking through the "shortcut" gates
Hopefully this helps
Because you have to delete the save data, it is advised you get all other trophies and just save this one for last.