USA passport, Japan alien registration card wants to visit China

currently I live in Korea, until February. I wish to visit Beijing in mid November for three days. I cannot obtain a visa from the Chinese embassy here because my korean alien card does not have 6 months left on it, and I cannot renew it. I am travelling to Tokyo next week, so intend to apply in Tokyo. 1. How many days do I need? , 2. do I then need to enter China from Japan, or can I still travel from Busan to Beijing?