to embark in a long and treacherous journey solo; literally, to travel one thousand leagues with a single mount (horse)

This ancient tale describes the renowned Chinese general, Guan Yu (關羽), during the Han Dynasty who traveled one thousand leagues to rejoin with his sworn brother, Liu Bei (劉備), while protecting Liu Bei's two wives. However, to get to Liu Bei, he not only had to battle the natural elements of weather, but he also had to pass through five fortified gates guarded by six enemy generals. He had no army with him at the time. All he had were his trusty Dragon Blade and his Red Hare Horse. This is also where the saying, 過五關斬六將 (penetrate through five gates and defeating six generals), originates.