Okay couldn't be assed reading all this ( sue me) but here's my tips for loosing weight or getting fitter.
1. Run , it's great cardiovascular exercise and you'll be doing a lot of it if you join the RMs. Learn to get over the pain and learn to run or quick march with a loaded bergan.
2. Swim , it works out your whole body , doesn't put nearly as much stress on your joints as other exercise and will improve general fitness ( also recommend if you want to be a marine )
3 Use body weight or dumbells and the like , not machines , you'll learn better form and you'll have to control your muscles more.
4. Push yourself!
If you want to loose weight something I've found useful is this.
Eat quorn instead of meat. Quorn is high in protein like meat but very low in fat , also in my opinion can be preferable in taste.
Hope that helps you.