The distance traveled on foot achievements seem to be a bit misleading as my achievement for traveling 100 miles on foot popped when I was on a 4-Wheeler.
Simply go to an outpost marked on your map, go to the locker which is located outdoors somewhere near the house and go down to where it says garage. The 4-Wheelers in here are free so feel free to get a new one whenever you stop back at an Outpost.
That's the fastest way to travel in the game, but if you want to kill the time a little you can do what I did and uncover all the Overwatch towers, Outposts and anything else marked with a question mark. You should hit 100 miles somewhere along the way as well as the achievement for traveling to every region if you uncover everything on both maps.