Applicants who look closely at their diet and fitness as soon as they have applied if not before fare the best, PF.
It seems to be a source of surprise to some applicants that there is a fitness test and if they don't take on board the need to pass it and put together a personal training plan, they can come unstuck.
If you are serious about your application, you should be starting to look at your fitness this week.
For what it's worth, if I remember correctly, you are still at school, which means that you should have access to sports facilities and advice from the staff, which should make the task a lot easier.
Yes. I am in the first year of sixth form and I go to the gym. I was always going to prepare a year in advance anyway.
I think that you should start the initial stages of the application process, now, PF.
You mentioned joining as an Officer. If you complete the various stages of the process this year, you will know whether there is a good chance that you will be going to BRNC after your A Levels and can then put your heart and soul into your exams in 2021. You may find that the different stages can fit into your school holidays. The AFCO will advise on this.
It would be useful for you to at least do the Expression of Interest stage now. Give it some thought.
Something I would counsel you against is leaving it all until next summer, as you may find that you won't then join until the following year.