I would guess - since this isn't my field - that the US and ROK Air Forces' strategy is probably something similar to what we had planned for the Soviets in Europe. In that case, we were also facing vastly superior (in numbers) forces. The strategy, as I understand it, was "deep strike." Basically, don't try to take on the heavily defended and armored combat units directly. Instead, take out the support tail: bridges, railroads, supply dumps, etc.
The front line troops have to fight a holding action or staged withdrawal in the face of superior forces. However, the attackers aren't going to get much further than they can go on one tank of fuel. Then with their restricted mobility and depleted ammunition stocks we would counter attack and execute maneuver warfare - attacking when and where we decided, with initiative and surprise.
What makes this problematic in Korea is that Seoul is only what, 40 miles south of the 38th parallel? Not a lot of room to trade for time.
One factor in favor of S. Korea and U.S.: North of Seoul used to be just another countryside. Not anymore. Heavy development projects made the region full of buildings and clogged roads. Such densely populated urban area, which sprung up in recent years, create a headache for advancing army. Buildings, especially apartment complex(many of them are made up of high-rise buildings) can be used as hand bunkers as defensive positions. Building can be demolished to create barriers.
Besides, there is far less flat area for providing armors and transportation vehicles to drive on. They will be trapped into a maze, taking fire from all direction.
Overdeveloped urban area can have some fringe benefit: to stop massed advance of enemy.:-)