Should US troops actually get involved in Korea in a big way, we've got a military that has recent combat experience, and a modern and (more importantly) battle-tested approach to war-fighting.
Pretty much we (and to some extent the British) are the only nations that can make such a claim. Certainly the North Koreans cannot make it. But we can (and probably have been) train the South Koreans to use what we have learned.
Again assuming that sufficient equipment and men can survive the first barrage, I have to think that a North Koreans attempt to mount anything like a conventional war, will have results very similar to what happened to the British during the first few days of the Battle of the Somme in WWI.
Granted, but I think Kim will be content to keep the bulk of his conventional forces behind the DMZ and try to blow up Seoul from stand-off ranges and protect the approaches to Pyongyang.
I could see him using his sizeable Special Forces and assymetric (read SRBMs) to wreak havoc nationwide.
I think the extent of a NORK invasion (realistically) would be a land grab for the 5 Yellow Sea Islands, and try to take and hold a 5-10 mile corridor south of the DMZ (ala the Egyptians in 73’) and sue for peace. Maybe he could make a very abbreviated/suicidal run for Inchon and Seoul . . . I think that is about the best he could hope for.