how do you mass 50,000 soldiers on the DMZ without either the US or south korea noticing? this is assuming it's only soldiers. if they used trucks and armored vehicles, they would be even more noticeable, and have to go through an even more heavily guarded area to avoid the mine fields and the ridiculous terrain.
so, assuming they have vehicles, the south will know immediately what's going and start bombing the ever loving shit out of the column of men. if they don't use vehicles, it will take them at least a day or two to get to seoul.
and this is all ignoring that there 20,000 US infantry garrisoned in seoul alone. and another 8000 airforce personnel located miles from seoul at osan air base.
this isn't even counting the south korean forces stationed there. and all assuming they get through the DMZ.