Definitely agree on the posture... walking though.. like jogging you get impact effects which can harm the back. Currently, I am biking and swimming has to be the best exercise because floatation takes alot of weight/stress off your back... mind you I'm not talking about 100m backstroke

Also I found that I can ski as long as I avoid moguls and crashing... usually my legs will tire before my back does and again you don't have the impact of walking or running. Eliptical machines are also good if you want the aerobics of walking without the impact, but hard if standing for long perioRAB increases your pain.
You can also look into recurabant bicycle exercisers. Basically you are laying back and pedaling. Now pedaling will impact your lower back as your quadricep muscles insert at your lower back, but for me cycling is helping. I just have to pace myself and start off slow and short and work my way up to longer times... but if you think about it pedaling is very similar to knee to chest stretches... Currently I have an electric assisted folding bicycle which I had to get a special exception to the local mass transit ADA policy so I can take it on the bus and trains using handicap ramps and spaces. As well as a change in the policy so I can sit while travelling with my bike.
(Standard policy dictates riders have to stand with their bikes.) It folRAB to fit in the same space as a scooter or motorized chair and the back support modification I had made, makes it too top heavy for the normal bike racks, but it does allow me to lean back and have lurabar support for my back.
But its also nice to know I am setting precedent for future people with back issues that want to use a electic assisted bike with back support. Because of my efforts, people who opted for the same means of transportation will have an easier time of getting the exception, since I put in alot of time in finding a bike that will conform to their space limit dimensions for a disability assisted device.