In the grand scheme of that area, the 28,000 is no more than a token presence, our main defense of that area comes out of Japan (air force) and naval presence (if any). Now it has been a long time since I was there but where were trained to fight (but basically a rear guard action) until help arrives. .
We have enough Navy, Marine, & Air Farce presence in Japan to shore up the 28,000 for a little while. Call it a Tier 2 defense. There's enough missiles on the DDG's out of Yoko to obliterate Pyongyang and any other vital target in the North. The Navy, USMC, & USAF aircraft would assist with anything moving on the ground. Okinawa based Marines and the Naval Amphibious Force out of Sasebo would get reinforcements onshore very quickly.
This level of defense would keep things going until stateside assets got to the scene.