When I joined the Army I weighed ~ 120lbs. Less than a year later I had completed basic training, advanced infantry training, jump school and the Ranger Indoctrination Program (programs completed in the order listed). After graduating RIP I served in special operations as an Airborne Ranger.

I'll never forget something that happened during my first training mission with the 75th Ranger Regt. After several hours of rafting and patrolling I was walking up a hill carrying a full load plus an M4. I was sucking hard, covered in sweat and my whole body was on fire. A tabbed E4 walks by me, not a drop of sweat on him, and up the hill like it's nothing. He looks over at me, as he's walking by, and says "you're carrying that and I'm carrying this." He was carrying a 240G (probably 5x the weight of my M4). I leave it up to you to form your own interpretation of his words.

~5 years in Regiment I weighed 170lbs and could run through everything I did my first deployment carrying twice the weight.

The importance of training is immeasurable.