Hi, long time exped climber / previously SF here, my thoughts;

Depends on the unit you’re going for as all selection course are different, often by a significant degree. You sound Commonwealth but I may be wrong, but within that there’s a lot of difference between units.

My basic thoughts are;
Selection is an aptitude test and ‘fitness’ is only about half of it as a physical thing, the rest is mental.
You need to be in good performance capacity but also free of chronic injuries. Capacity needs to be good an general re strength (not strong at the expense of other areas, just strong in a durable kind of way), and you need a serious weight-output ratio. What I experienced was skinny but strong did best overall.

Endurance-wise you need to be far better than average, but on your feet and for repetitive actions, so all your Z2 stuff pays big there, but spread it over your whole body not just you legs. If you can’t just move for 18hrs a day it doesn’t matter how strong you are, so get the volume before all else.
I get the idea of ‘on the day’, but staff will nullify that very advantage, so by having several reference sessions of huge days you will have a big advantage of those who don’t.

Your training needs to get your head in the right place and that means stress management or ‘inoculation’. Every course very fit guys pull out in the pool, the fast runs, the dark and the weird stuff and those things are in the course exactly for that reason. If there’s a weakness they will find it and how you deal with it is how you get assessed.

Remember; selection is looking for guys they can work with and that means showing you prepared well. 200 pushups but a shite 2 milers shows you love the gym but can’t run till you spew which matters more.

You quote this;
>>>
1. Overuse injury Stress fracture/Tendonitis etc.
2. General recovery between events
3. Poor nutrition and drained energy due to enforced reduced diet and sleep deprivation
>>>
Well that’s exactly what they’re looking at. I prepared with PT sessions with reps in the thousands, moving with a pack with good shoes (don’t believe all that bullshit about training with your boots on, it only injures you. Wear in your boots doing easy stuff), learning to find my best zone to work in to avoid burn out (you won’t recover well after a max effort) and training through the night. I used no supplements from about 5 months out.
They say you can’t adapt to sleep dep, and maybe you can’t – but you can learn how you react so you can manage it. Same goes for water, cold and boredom. I saw otherwise fit guys literally lose it because they thought they would go crazy.

This is getting long, but in the end, the guys (and girls) who do well are those who love it. 75% won’t be about your fitness, it will be about your character, so make your training reflect that. The guys I saw that were only fit got targeted and had their heads done in by staff who had seen it a million times before.

Oh, and learn to sing with confidence, a few good jokes, a dark sense of humour and how to vomit and move on.

I won’t say good luck because if it’s about that you’re in the wrong business.