It used to be easy!
I've been known to walk a good couple of miles fully loaded round the reservoir or lake.
Fully loaded with water that would be 100kilos, but spare clothes, waterproofs, stove, cooking equipment, and scales etc only weigh about 40kilos.
The 40litre Molle packs are available if you have a search through the surplus stores, and from personal experience they last.
The various specialists are looking to the conclusion of Long Covid having caused it.