It's hard to describe how badly you have to do and I think it's altered depending on the person too. For example when I went on selection they entered all sorts of details like my age, whether I smoked, whether I was subject to passive smoking etc. Therefore it's hard to tell you, personally, how well you have to do.
The only thing you can do is go and do it, don't get worked up about it, and if you don't have any problems you'll get through fine. If you fail, it's because you've got a problem that makes you unsuitable for life in the army. It's a cnut, but that's just the way it is.
Cheers for the reply matey. So myself confessing to the occasional fag and having a few smoking pals all affect what I must get, is there any other stuff? technology these days hey. pretty sure i dont have any worries, i can breath (just).
I think if you're working on cardiovascular fitness that will help (although that's just an educated guess), because you do ten minutes on an exercise bike and do a repeat test after that to see how much your breathing has changed. If you've done your best to get fit, it won't change much and you'll have more chance of passing (I assume)!
It's a big deal if you don't pass it within 8 go's! I struggled to pass within 6 but that was because I stupidly smoked the night before and I'm only a social smoker. My advice would be not to smoke for at least a week to clear your lungs.