Start at the Blackwater Trainstation (eastern most part of your map) at 0600
the fastest and easiest way to get this is to follow the railroad tracks, for some reason the horse runs faster, also you dont have to following pointless winding trails. whenever the railroads fork, just take the left hand track keep following the track until you pass Chuparosa once pass the town keep following the tracks until you see the bridge that goes back to the US (DO NOT CROSS THE BRIDGE) before the bridge get off the tracks and head to Escalara.
once you enter the town the achievement will pop.
if you leave at 0600 you should arrive at the town around noon.