The other solution is physically the most efficient, but if you want to optimise your XP gains, the best option is to wait until level 5 to unlock the Lawbringer I challenge then do the hideouts in the following order:
Pike's Basin
Ft. Mercer
Tumbleweed
Gaptooth Breach
Tesoro Azul
Twin Rocks
Nosalida
That will also complete the Lawbringer I challenge and net you some extra XP for each level/hideout.