Walking shouldn't hurt your back and its very worrying that it does, you should look into some resistance exercise.
so i started yoga cos another friend of mine recommended it after i 'seized up' my back suddenly..it happened like innocuously me trying to pickup something from the floor..i can feel a pull and im like oh shit this is gonna get worse and i cant move properly for a week kind of thing. and it does. i blame it on potatoes and cauliflower cos i seem to get it a few days after i eat them dishes and lot of sitting and reading and programming. luckily it does go away in a week or so and im like as if nothing happened. but going thru it feels like im bugg3red.
cycling alone doesnt cut it i guess.
A friend of mine spent most of a year having intermittent back pain and it turns out that his problem was that he had developed fructose intolerance. His pain was not coming from his internal back as he thought, but from his inflamed intestines, which happened very frequently since fructose is present in a lot of foods.