But otherwise, why is doing small amounts at home of what I am going to do out in the bush anyway bad for me? I keep following to the logical solution that this suggests we shouldn't walk with packs at all because that will bugger knees / backs whatever.
It'll only cause injury if you carry too much weight &/or exert yourself beyond your physical capacity. Within reasonable limits regular & proper training should address both of these.
It seems everyone is concerned about me doing 30 mins of exercise in a room with a 20kg pack, but not that I am going to be doing that same exercise (with more variables to go wrong) for 10hrs a day, 8 days straight in a couple of weeks.
Doing the exercise at home should be fine, personally I'd be more concerned with jogging whilst wearing a weight vest or fully laden pack. The weight of the vest isn't much but landing with each step causes a sudden deceleration that produces much greater forces overall. But, there are people who do this, again, with no ill results.
Also, if done slowly 20kg is a very very small weight for a grown man to be lifting, I'd be shocked if you hurt anything assuming you aren't
running with that weight on your shoulders.
I don't see my joints as something I plainly consume over my lifetime, so I should ration them for a few goods walks and baby them the rest of the time. My understanding wass that they can be protected, and deterioration slowed and mitigated with good exercise.
This is also my understanding.
I have to exercise a few times a week minimum otherwise I start to get problems, I'm guessing this is de to having a desk job.
In fact there's quite a bit of research in support of your understanding (not an expert, but it's a subject of interest):
https://en.wikipedia.org/wiki/Utah-Para ... physiologyI believe pre-existing injury is an exception to this. If you've damaged something bad enough any further damage takes much longer to heal & has a more cumulative effect than it'd normally do.
With the above in mind I'd say striving not to injure yourself (especially backs & knees) is very important. That's a fine balance between sufficient training & not overdoing it. I honestly have no idea what that balance is (again not an expert). Everyone here seems to agree that regular training is vital, the argument seems to be about what level is "over doing it". Personally, at least with my knees I err on the side of caution.
In my personal case my left knee is a bit dodgy, if I do nothing at all it goes to hell in a week. If I try jogging 5 days a week it'll also cause me trouble, regular walking & hiking seem to stave of problems. I've recently tried trail runners though & they help a lot, 2-3 days a week of running seems to be a sweet spot between maintaining basic fitness & not causing things to get sore. This is all anecdotal though.
In any case I'd love to have an expert chime in, I've spoken to a phsyio about this kind of thing (join & cartilage adaptation) when I was having rehab a few years back, but training for hiking & the spinal column never came up.