Yeah, typically when you go see a recruiter, they realize you are not in condition--so they put you on something called the "Delayed Entry Program" so let's say you need to lose about 20 pounds and get your cardio up to snuff-- they will have you report several times per week, or maybe once per weekend, and you will begin to train and discipline your body and mind to handle the rigors of Military.
But if you change your mind, they will try to scare you with all sorts of threats that aren't real like "You'll never be able to even get a job at McDonalds" etc.. (I'm specifically referring to the Delayed Entry Program) but yeah, once you raise your right hand at the MEPS (Military Entrance Processing Station) and then quit--hoo-boy you WILL be in a heap of trouble. So like the Bible says "Make your election sure"
Me: 6 Years Air Force (F-15 Crew Chief)
then 6 Years Army (Air Assault Infantry 1-506th, 101st Airborne Division (AASLT) )
It's rough on the body--especially going into Army in your 30's woo--I regret that one.