If North Korea invades, all bets are off - there simply isn't the manpower anywhere near to stop them before Seoul. Those 35,000 or so US troops would be little more than speed bumps to the million man army (with millions more in reserves) which is on the border right now.
Right now the entire United States can muster about 12 ground combat divisions - tops. We've decimated our military so greatly that even winning a second Korean War is doubtful.
Another good point. The North Korean military has 1.2 million active duty, with nearly a million on the DMZ, but that's just their invasion force.
As reinforcements, they have several organizations. The Reserves are a full 800,000, and can be used as maneuver units. The Worker-Peasant Red Guard militia at around 3.8 million total manpower, but they are more restricted to their local area. Also, the Red Youth Guards have roughly a million under arms, with reasonalby good basic infantry skills. Sprinkle on another 120,000 or so security personnel, and you have a whole lotta people under arms.
The most it can do now is defend fortified positions. And most of North Korea is a fortified position - they've poured enough concrete in the past 40 years to make Japan's Yakuza green with envy.
And they can fire artillery at pre-planned targets, i.e., Seoul.