Tread mill seems a proper fall back for yeh with the conditions you posted.
But there's other work to do too. You've listed the hi speed, low drag options in your choice picks.
You want all over endurance. You want toughness, you want mean that you can dig into when you're too tired to fart and too whipped to think.
Calistenics, lots of em. Bends and thrusts, mountain climbers, getup getdowns, sit ups, crunches, leg lifts, push ups, crunches, leg lifts, pushups, side straddle hops, push ups, leg lifts, high steps (hands behind head, bring left knee up to meet your right elbow coming down and across, then right knee to meet left elbow, repeat quickly, even faster, like you're running in place till you want to puke) push ups, leg lifts, sit ups, push ups...did I mention leg lifts and push ups?
Also, sit down and grab your nutsack and self check. Do you have it in you to be miserable, go without sufficient sleep and food for days on end and not only keep focused and doing your job, but not whine like a ruptured c**t? How do you feel about still being awake at 3 AM and looking at yet another 10 mile hump before you get even the possibility of an hour or so sleep, for the 3rd day in a row?
Life is hard even without making the choice to live the hard life. This aint about ego or what looked cool on TV. You'll be in a world where the kind of tired you'll feel dont even exist in the civilian world. And, while you're dealing with all that soul breaking fatigue, pain and misery, you'll need to be sharp, focused and "on time, on target" all the time, every time.
It's not a bad thing at all to want to go hard, but you dang well better understand that if you dont make the cut, you still serve out your contract.
