Completely new

Newbie

So I am completely new to running, I havent purposely ran a mile or more in my life.
I need to do some training or pre-training, however you wanna look at it, for the Military. I thought this would probably be a good place to get information and ask questions.
The main one I have right now is Is there a limit or threshold as far as safety and how much pain should I experience??

I am in terrible shape atm but luckily the road I live on is a great place to train. It isnt paved and has some loose gravel but its almost a perfect 1/4 mile. I ran 2 miles on it yesterday. After 2 1/2 or 3 laps I had to stop and catch my breath for a minute or so. I was experiencing alot of pain in my lungs but I continued through it till I was done with at least 2 miles.

I realize pain in any muscles, lungs, and being able to feel my pulse through my whole body is normal but to what degree?
Since I just started is there a point where I should stop?? Its was a little more than I expected it to be but its no problem to go through it, I just wanted to know if there was a point as a beginner I need to stop.
Within the next year I need to easily be able to run a 18 minute 3 mile and have my cardio keep up with me all day long.

Thx everyone for any help.