Found this on T-Nation. This guy is former SWCC. Read up, and Live it.
Myth: You know how much stress your body can take.
Mythbuster: Craig Weller
During Special Operations selection training, you're subjected to a brutal series of physical and mental tests. Depending on the program and the time of year, between 60 and 90 percent of candidates won't finish. Fun stuff.
But it taught me something important: Pain does not stop the body. There's nothing that hurts so badly that you can't keep going just a little longer.
Extreme and continuous stress teaches you to break daily life down into short, measurable goals. You make it to breakfast, and then you focus on making it to lunch. Sometimes your mind refuses to project beyond the immediate future: running one more step, swimming one more stroke, grinding out just one more push-up.
Everybody hits bottom at some point. You get to a place where you'd do anything to make the pain stop. If your mind breaks first and you stop running, or wave for a support boat on a swim, or raise your hand during a beat-down to say that you're done, you're officially "weeded out." You've quit. You're part of the majority, but you still feel like a loser.
Fortunately, there's a loophole: If your body breaks first, they won't hold it against you. Every guy in my squad had the same perverse thought at some point: "If I can just push myself hard enough to black out, I'll crash in the sand, take a nap, and wait for the medics to revive me. I'll get a nice little break, and then rejoin the pack."
So we ran harder. We pushed. But we hardly ever got those naps.
I remember being on a run, soaking wet and covered with sand. We'd just gotten back to our feet after calisthenics in the surf and a series of sprints up and down a sand dune. Then the instructors took off sprinting again.
I didn't think I could make it any farther, but I knew I could never live with myself if I stopped running. So I put my head down and sprinted as hard as I could through the soft sand. Pain surged through my body, and the only conscious thought I can remember was that the air I was gasping into my lungs had turned to fire.
I focused my eyes on the heels of the instructor. The pain was getting worse, but I kept going. I could hear another member of my class behind me, struggling to keep up with the pack while puking between strides.
Guys who went through the training with me had similar experiences. They'd hit bottom one day, and think they could finally reach their breaking point if only they pushed a little bit harder. But it never worked. The agony would only increase. But so would their capacity to keep going. Pain, in other words, never actually broke our bodies.
Which isn't to say we weren't incapacitated from time to time by hypothermia, hypoxic blackout, hypoglycemic shock, or some other things you find in the dictionary a few pages past "hell." But passing out was acceptable. Quitting wasn't.
I'm a civilian now, running a facility and training people. Every now and then, I hear someone say, "I can't."
Frankly, that's bull****. Next time you're tempted to say you "can't," remember that what you're really saying is, "I don't want to."