So we teach basic military skills and fitness in school. At least then you aren't starting from ground zero. If you look back the the Vietnam era, the USA ramped up their troop numbers at a massive rate and relatively quickly.
Reinstating Nation Service akin to the post war years will bankrupt the country, so what are we going to be left defending at that point? However many troops we have they aren't going to stop a missile hitting one of our cities, and they certainly aren't going to be in a position to do anything much about the aftermath of an ICBM.
As for weapons manufacturing, we aren't talking about conventional warfare with dumb weapon systems which you can knock out relatively cheaply and quickly. Yes, you can have a load of dumb weapon systems, but they are next to useless against an adversary with anything relatively sophisticated or high powered.