Sorry Brown Bear, this bear disagrees. :swordfigh The most essential bushcraft tool is the mind.
I'm ex forces and was superfit. Now, I've been out a while, I'm 40 years old and on the large size. Is my enjoyment of the outdoors compromised by not being superfit? Not at all. I've just gained more knowledge, slowed down, and enjoy the chance to look around me, rather than seeing the blur of the outdoors zoom past me as I tick off another distance/speed goal.
Some things I'll take longer over. I've no problem with that.
One other thing about the mind - you can use it to work smarter, not harder. Convince the younger, fitter people with you to do the harder stuff. I'll take it easy and tend the fire.
Before I clicked on the thread to see what everyone was saying, this answer popped in to my head, and I thought more people would have mentioned it. It's not even so much the saying "the more you know the less you carry" either, it's just about having the common sense to treat your surroundings with the same respect as you'd give any other place- if not more. It's about being able to relax and enjoy the moment, but being mentally prepared to step it up a gear if needs be.It's about trusting those around you with more knowledge or experience than you, or being confident (without letting your ego take control) enough to offer advice, if needed.
However a certain degree of physical fitness is needed for certain things, but as long as we know what it is we can do and don't over do things too much or worse under do things then we can all manage to muddle through. The last part there I've saw so many times in some people, mostly much much older. They slow down due to a little pain, here and there and over time the pain takes further control letting them manages less and less.
Please let me say I am in no way inferring that anyone here isn't trying to do their best, this isn't made at anyone here.There are many conditions that simply cause too much pain to be able to do things that most of us take for granted, my ex was one such person, who couldn't even eat or talk without causing so much pain it would make most of us cry out. I know a man of around 55 who is always complaining of this and that, going to doctors to get sick lines for sore elbows etc, when there is nothing wrong with him the GPs say that so he complains more, the consultants say it and he goes in a sulk. To look at him, you'd think he was 75. On the other hand ol' Jim down the pub takes a bunch of oldies , as he calls em hill walking on regular occasions. Takes his grandson and his mates hill walking, canoeing and camping. He's up the Cairgorms several times a year, and generally lives life. He's 83 this year.