I am writing a fantasy book and would like to base one of my cultures on Mongolian horse tribes around the time of Genghis Khan. I have gotten a lot of good info from the internet but would like to know more about diet, interaction between tribes, how far they moved and how often, daily life, hierarchical structure, and more. Is there anybody out there that could help me or point me to some good resources?
You're in luck. The AskHistorians Book List has a whole section on Mongolia. I can't say for sure that these will provide info on daily life, but they should be a good starting point if you don't get more responses.
Also, just to nitpick, I don't think Genghis Khan's time would be considered "ancient" Mongolia, since he died less than 800 years ago and Mongolia has been occupied by humans for thousands of years.