That's interesting about walking. I've done longish walking pilgrimages lasting several weeks (Camino etc.) and some stomach problems and joint problems improved a lot. I usually walked about 25km a day - I realise that's longer walking than what you mentioned.
There are some books about walking putting illness into remission. A famous one is "The Salt Path" where someone with "corticobasal degeneration" brain disease was positively impacted by their walk. (Although the claims are in doubt now because the main author wasn't truthful about other aspects of their walk)
I wonder if more light exposure during walking is also a factor? Many folks don’t get enough vitamin D, and light therapy for SAD and other conditions has shown promise as well. I’ve also noticed that 25km+ of daily backpacking for a few weeks continuously helped my overall fitness and wellness, though I don’t have any chronic conditions or ailments at all to speak of. I do question whether many folks would invest the time and effort to do the work, even if they desire the benefits of the exertion.
There are some books about walking putting illness into remission. A famous one is "The Salt Path" where someone with "corticobasal degeneration" brain disease was positively impacted by their walk. (Although the claims are in doubt now because the main author wasn't truthful about other aspects of their walk)
Anyway walking probably a real positive overall!