Lets see well from what I remember if correctly the Mongol System was based on the Yassa which was essentially a Meritocracy like system that set up society and warfare.
Ghenghis Khan arranged his army into Tumans or 10,000 strong units. He made sure that the Mongol Warriors were ready for war especially with requirements for remount horses and so on. I thinkpart of their trainning and effectivness involved their lifestyle, they practically lived for the hunt and warring, with things such as horse riding and archery (which they used to dominate all) was essential to life on the Steppe. Plus with this more are able to go into action as theya re not constrained by sedetary living.
As the Mongols marched westward they would absorb numerous Steppe tribes into their Empire, very culturally diverse, the empire was pretty muchrun by the peoplethey conquered.
Further the Mongols were so successful because of their own logistic system, the complx nature of it allowed them to respond to attacks thousands of miles away in quick time and condct their complicated hit-and-run, feign attack, and manuvers they are sofamously known for. During Ghengis Period, he put absolute trust in his generals such as Subutai, and they in turn put their loyalty to him and it would be essentially them who would conquer the greatest extent of the empire.