Sorry guys, I know this has nothing to do with CVAs.
WEBF
If you have injured your knee get professional medical advice and see what training you can do on it. It doesn't mean you stop, work on other parts of your body, I am sure in basic training you will be running around with large backpacks at some stage - the army and RAF do!
There is a poster up in the gym at my UK base. It shows a trainee Royal Marine doing chin ups with a leg in a plaster cast, with text saying something to the effect "We don't let a little thing like a broken leg stop us in the Royal Marines". OK I realise it is appealing to the macho side of it's audience, but what it shows is COMMITMENT.
I have an "injured" knee at the moment. I am at a "secret airbase" in the desert somewhere, sharing limited gym facilities with 600 odd people. This means that the gym is only quiet at unsociable times like 11pm+. However, I am still going every other day (I am flying on alternate days at a rate about double that achieved in the UK), and using the equipment that puts the least strain on my knee, bikes, rowers, steppers etc, to say nothing of upper body work and weights. I am not a fitness freak, or a gymaholic, just an average squadron guy in my mid forties. I do not have any DS to impress or a course to pass, I am already in the military!!
You need to make some decisions about your level of commitment if you really want to be in the RN. The RN doesn't owe you a place, and does not measure commitment at your stage of the game in terms of how many letters you write to your MP trying to save the Sea Harrier. Put the effort in or admit to yourself you will never actually be in the military and get on with the rest of your life.