Khunjrab Pass is the entry to Xinjiang Province from Pakistan and the nearest city is Kashgar. That will be a long long way no matter by bus or train. No direct train and bus at all need transfer for several time. One day from Kashar to Urumqi, aboout 49 hours's train from Urumqi to Chengdu, then over 30 hours' train to Nanning then. MOre transfers and time are needed by bus. So, I suggest you should fly, at least part of the route.