Going through infantry officer training you need to be physically fit for starters.(My course 37 started 12 passed, what fun)
We would run 8 k in under 40 minutes every morning, stopping to do upper body just long enough to tighten up the muscles. Usually you are running on 2-4 hours of sleep a night.
You also need to be able to carry moderate to heavy loads, and have endurance ... ie digging, carrying, marching, and then to top it off THINKING in a fatigued state of mind.
A lot of the failures are things breaking - knee injuries etc. Of all of us who passed, I think we were all hurt. It's just whether you ride it out or not.
The technical bits aren't too hard - writing orders, navigating, radio procedure, assigning tasks - the trick comes down to three things: physical, mental, and emotional stamina.
Physical: If you are weak to start with, your body will break down and you will be more prone to injury, illness, or you will collapse. GET FIT BEFORE YOU TRAIN. If you are suffering physically, everything else goes downhill.
Mental: Lack of sleep, mental stress (being yelled at, threatened with failure, given tight/impossible timings, given too many tasks), lack of experience will wear you down. This is deliberate.
If you are physically fit, it is easier to handle. Almost always if the body is working, the mind is too ... if the body breaks down, the mind follows soon. BE PHYSICALLY FIT WHEN YOU ARRIVE.
Also, previous experience with mental concentration helps. It's like driving eight hours on a dark highway in a blizzard. It's not that hard in itself, but you have to be constantly alert, for a long, long time, and it wears you out. If you slip up you crash.
If you lose your mental edge, you will spin out of control, and freak out on fellow candidates for not polishing the urinal or other minor crap. Yelling, voice cracking, eyes bugging out. It would be funny if it weren't so sad.
You can see troops start to unravel a day or two before they are completely unwound and either quit or fail a test, or come up with some excuse why they have to leave.
Emotional: Don't be a quitter. Many who fail allow themselves to fail - exaggerating injuries, or convincing themselves 'the course is b.s.'. So what, it's a few weeks. If it's b.s., pass it, then bitch about how stupid it was. If you pass, you have your qual, and you won't care much anyhow.
So, will you be one of the failures? If you are physically weak, mentally weak, or emotionally weak, probably.
If you decide NOW you WILL pass, you probably will. But that translates to getting your butt to the gym, pushing weights, working cardio, and TELLING yourself the only way you are going to fail is ... you ain't. Get mentally tough, visualise success.
And if you hurt yourself legitimately, and you don't wuss out at the first blister, you'll get another shot. (ie if you break a leg, or get pneumonia, or something). The staff can tell the difference.
So, will you fail? ... it's on you.
But I never saw a tough, determined, moderately intelligent candidate wash out, unless they got a really crappy break and got hurt, in which case they came back. Or sucked it up and finished like a walking zombie.
If you want some concrete answers, contact the school, and ask what kind of PT standards they have, and EXCEED those numbers by a longshot before you show up.
And throw yourself down a flight of stairs every morning. ;D