It might be a good idea, I'd maybe join a few more while you're at it! When you go through selection you are tested in 10+ sports (football, cricket, basketball etc) and required to instruct an aspect of a sport as well (serving in tennis). You also need to be a brilliant swimmer, I mean perfect stroke technique, as you'll one day have to instruct this too. I would make sure you're practising you're mile and a half run on a treadmill as this is something you'll be tested on early in you're application, and the better your score the more the more competitive you're application will look. You should also be getting a competitive press ups and sit up score. Bleep test is another big one they like to test you on, and again the higher you're score the better you look. A PTI I know regularly gets 13 and he's 40, think accordingly!