For the last 15 years I found that if I was standing up or walking for over 40 minutes I would begin feeling lower back pain. If my uprightness continued for hours then the pain became chronic.
Then I did something several years ago which in hindsight turned out to be very unwise - I mixed and poured 5 ton of concrete with a shovel alone over 4 days, using incorrect posture and working through the initial back pain until I was numb to it - this just made things worse.
It got to the point that I was no longer able to enjoy my long walks, it even became an annoying hinderance trying to sleep.
I spoke to someone who taught Judo and he said my problem was caused by some of the muscles around my lower back being very weak - all due because my job involves lots of sitting in a chair. He gave me some exercises to do but I didn't find them to be of any benefit.
Then I stumbled across an article on the internet that suggested an easy way to alleviate this pain was to tuck in your stomach like you are tucking in at your belt buckle, just a little bit, only enough to feel a little bit of inward pressure - and it worked for me. Whenever I find myself upright for long periods of time and start feeling that numbing pain on my lower back I apply this technique and find I can alleviate it.
I'm back to walking long distances again and the hinderance to sleep has mostly gone.
Unfortunately I still want to pour another 2 ton of concrete at the rear of the holiday house - been putting that off for years.