The fact that American boys are increasingly preferring video games to sports should also worry you. There is a shortage of fit young American men for special-ops programs like the SEALs, etc.
The USA, technologically and manpower wise is doing fine at the moment in addressing these external threats.
You should not worry about the fighting ability of the average Chinese man vs the average American man. U.S. has the most violence-ready population of young men on Earth, and it's not close.
Maybe Latin America or parts of Africa and the Middle East beat us in terms of willingness to commit violence, but we have much more resources, training, and equipment. China has none of this.
In fact, you should be worried about our young men choosing to exercise this violence against you here before you worry about some kind of military defeat at the hands of the Chinese.
Can confirm. I have seen South Park.