You guys are underestimating the abilities of the ROK Army, as I said. This isn't the same army of 1950, not only are these guys properly equiped but they have a real motive to defend South Korea.
Yes, the North Korean Army is large. A 1.8 to one advantage is very helpful but unless SK intelligence (and US for that matter) are profoundly stupid the gear up for conflict should be pretty easy to see. Every bridge in the North is targeted, you can be sure of that. Slowing down the NK army even a little bit will probably cause them to peter out about halfway down the peninsula. Worst is probaby a Pusan like perimeter. If the US becomes heavily involved (which they will, nothing like killing 20 thousand US troops in an unprovoked attack) then look to see another reversal like in 51. However this time don't expect China to come to the aid of the North.
The North knows this. The best time for NK to attack was in the mid 70's, when US morale was relatively low, the US was still in shock from the ending of Vietnam, and the ties between NK and the Soviet Union and China were still relatively strong. Now they don't have the food stocks to maintain an army in the field, nor I bet spare parts or replacement equipment. They are strong, but not as much so as an offensive force. Offensives, especially against well dug in enemies like the South (remember, these guys have had 50 years to get ready for them-I have seen the prep work) use up a lot of men and materiel.