I'd long known about the whole thing with military training and heavy packs, but it's a whole thing in fitness:

https://3vgear.com/blogs/adventure-log/how-to-get-fit-while-rucking

Having spent a lot of my time homeless carting around 10-15kg on my back, I had sorta forgotten what it was like, until the other day, with 8L of water in my pack, and again today with 12L.

I need to get a better pack at some point but the 25L I have at the moment is adequate. The only thing I need to do is put a cushion at the bottom where the bottles currently jut into my lower back.

Combining rucking with barefoot shoes, it's a weird but cool feeling, walking properly on the ground, with this huge weight on my back. Really feels like you are pushing the ground behind you, if you know what I mean... haha.

I'm gonna do 12kg every other day for a while, eventually I want to be able to put a full 20kg, and then do my 3 mile circuit at a gentle 8km/h, then I'll be fit.

The other thing I'm doing, just started today, is pull-ups. Tried to do some with the 12kg on, couldn't lift my chin to the height of the bar, nowhere near it.

Gonna be a really good feeling when I have 20kg on my back and I can do even one pull-up like that.