If this is a career you truly want, and the RAF is a path you are considering, the fact that you are not fit now should not be seen as a barrier to entry; rather, it is a challenge for you to overcome.
You can still apply to the air force up to your mid 20s so if you are young that is plenty of time to get fighting fit. To overcome an injury would be a great demonstration of determination to discuss at interview! Join a local sports team to combine fitness with improving your team skills.
The RAF is a lifestyle as well as a career so make sure its for you, and if you are eligible for selection give it a shot! Or as many shots as it takes!
In the mean time; you don't need to be a pilot to work surrounded by aviation. Perhaps there is a local airport you could work at? Anything to get closer to your interest and passion. You will enjoy work more and are hence more likely to perform well enough to one day fund a licence.
Wish you the best of luck!