At the risk of slipping into seriousness, you cannot create a single ethos for fitness throughout the 3 services.
The Army will always be fitter, it is what they do. The infantry train like madmen and the other units have a soldier-first mindset.
The Navy and the RAF are sailors and aviators/technicians first and always will be. Fitness is important but technical ability and professional knowledge are more important.
Trying to ram home the Army culture of physical fitness will not work in the RN and RAF. Churchill said something along the lines of "fighting air groundsmen" at the same time the RAF regiment was set up in WW2. Fitness should be role-specific although there should be a reasonable baseline. If the MoD think that Bloggs the techie should routinely pick up a rifle and man a sangar or be ready to march with full PLCE then they are flirting with disaster. That is not his job - although, in extremity, Bloggs should be able to save his own skin and maybe that of the man next to him.