I am in Thailand, I have an F visa from China that expires 8-21-13 What is the best way to extend?

I have a passport good yil 2017 , presently in Thailand after attending a tefl school and may go back to China to teach English and/or tour. My Chinese F type expires 8-21-13 but it says that is also the "enter before date." If I enter before then Iam I good for 90 days to prepare Z type? I don't plan to return to USA