Is it possible stay 72 hours in Xian? to hike Mt Huashan?

We're our Vietnam trip in Hanoi and are trying to get toXian. We found a from Hanoi to Bangkok, and then a round trip from Bangkok to Xian and back, with a flight from Bangkok to Chang Mai to catch our flight back to the US.

Can we leave the city of Xian with the 72 hour rule to hike Houshan in the same province?
Because its round trip from Bangkok and back, then catching a flight to the US out of Chang Mai, do we still fall under the policy for the transit 72 hour rule? Any and all info would be helpful.