This is for maxing out Gladiolus' Survival skill.
Gladio's skill accumulates skill points from steps taken so any time on a chocobo or in the Regalia will not add to the skill.
I wouldn't recommend grinding this skill till you have finished all other trophies as it should be at at least level 8 by the time you have all others even if you have been neglecting walking.
The current estimate stands at ~30 hours of walking is required to max out.
The best way to force this is to head to Hammerhead and use an elastic band to pull the joysticks of the controller to the centre and leave the game to run as you go about your life.
Keep in mind the trophy won't pop until you go to sleep and it levels up.