Yeah I know I think its mostly down to the fact ive been smoking from a really young age and for the last 5 odd years I havent been doing any exersize until i applied.As mentioned above it's well structured by people who know what they're doing so as long as you work hard you'll get there in the end.
However If I had my time again I'd certainly of done a bit more training before I arrived, I met the requirements but only just and as such i found it a struggle at times, you'll have enough to concentrate on without having to worry about PT. The fitter lads on my intake only had any worries when it came to battle PT or bayonet training which would exhaust an Olympic athlete anyway.
That 5k time is a bit worrying for someone your age about to join the infantry, I'd be looking at shaving a good few minutes off that. Look at varying your workouts to include some muscle endurance as well as cardio, a martial art or circuit classes would do the trick. (goes without saying to train smart at this stage regardless of what you do, the last thing you want is an injury)
Enjoy it and good luck!