Dig a hole in your back yard
Up Front, Bill Mauldin, 1945
Dig a hole in your back yard while it is raining. Sit in the hole until the water climbs up around your ankles. Pour cold mud down your shirt collar. Sit there for forty-eight hours, and, so there is no danger of your dozing off, imagine that a guy is sneaking around waiting for a chance to club you on the head or set your house on fire.
Get out of the hole, fill a suitcase with rocks, pick it up, put a shotgun in your other hand, and walk on the muddiest road you can find. Fall flat on your face every few minutes as you imagine big meteors streaking down to sock you.
After ten or twelve miles (remember, you are still carrying the shotgun and suitcase) start sneaking through the wet brush. Imagine that somebody has booby-trapped your route with rattlesnakes which will bite you if you step on them. Give some friend a rifle and have him blast in your direction once in a while.
Snoop around until you find a bull. Try to figure out a way to sneak around him without letting him see you. When he does see you, run like hell all the way back to your hole in the back yard, drop the suitcase and shotgun, and get in.
If you repeat this performance every three days for several months you may begin to understand why an infantryman sometimes gets out of breath. But you still won't understand how he feels when things get tough.