F, ex-RAF Engineer Officer, happy to answer general questions.
As an ex-RAF Recruiter as well, I can say that the vast majority of people don't train their whole lives to join up. What they have done, and not necessarily with a particular career in mind, is that they get involved in activities. They do societies and clubs at school and uni, scouts or cadets, charity work, mentor, volunteer, play sports. And what's more, they take on responsibilities within those activities; they are life's natural doers and organisers and leaders. You've got something of an advantage here with your job, but probably there are other things you've done and are doing that can show the 'leadership potential' the RAF looks for.
Ultimately, it doesn't matter what grades you have or how many sit-ups you can do above the pass rate; if you meet the minimum requirements listed on the recruitment website, you are suitable to join and during selection you aren't judged on those things anyway. There's a whole raft of other tests and there's no telling who will get through or who won't. I was at OASC with the daughter of an RAF officer and she failed miserably at interview, and I've seen graduates fail the AST and those who've come out of their shell since Filter Interview to win awards during Basic at Halton.
if you are good enough you'll make it and the competition doesn't matter. You only ever see a small percentage of who you are up against anyway, so whilst you need people on board with certain of the exercises, you concentrate on how you are doing and that's all that matters.