One of my best friends just got back 6 days ago from the Marines.
We live at almost 7000 ft and he was a cross country runner. I am giving you this background because he had a slight advantage oh good physical shape that others might not. He went to San Diego which is about 0 ft. It was hell. Other than the hiking up steep hills 25 miles some days (During the Crucible) and the other intense physical duties like hundreds of pushups situps etc you have almost evil DI's screaming in your ear from about 6 inches away all the time.. About everything. He said the games they play with you are hard to handle.. The "gas chamber" sucked (tear gas, w/o the mask). The Crucilbe was a week long, 12-16 hour days of hiking and running up hill, almsot no food. Cold nights and early mornings. Overall he said they kinda lie to get you in, its not what its cracked up to be, its harder than he thought, but he could not be more proud to have accomplished what he did. It was hell, but I imagine it feels pretty dayum good to complete such an undertaken. He has changed, and I really respect what he's gone through. It will make you a better man... however get in the best shape you can before you go !