Think you're taking this a bit too seriously. Firsly, they're not a branch of the armed services, they're a regiment of the army. You don't 'join the SAS', you serve 3 years in the regular army then put yourself forward for selection. During this time you will realise that they are in fact just like any other regiment, just with SF roles during operations. They require the same services as everyone else in the army e.g. logistics, medics, intelligence etc. so they still serve alongside everyone else. Although their training is especially tough, there are many other units in the British Army which have insane selection processes, pathfinders, forward observers and so on. I'm not saying for a second SAS selection isn't tough, but it is overly glamourised thanks to Andy McNob and so on. If you are serious about attempting selection you train for months and months, then the selection course itself is designed to build gradually on your fitness over many weeks, it's not a case of being built like Rambo from birth.