Sounds like a good plan. The more you walk the stronger your back will become also.
As we age we often tend to let things slide and before we know it ten years have passed without doing any real exercise. Just watching children play it is amazing at how they twist their bodies, their ankles, joints, legs, arms, everything is moving and going in different directions. This is what a healthy body can do. When we have become really out of shape we have to go back to basics; try doing jumping jacks sometime or a push up. When you see athletes warm up they do little running exercises while scissoring their legs (which twists their waist, etc.). These little exercises allow us to regain the flexibility and ease of a healthy body enjoys.
Walking is great! Just don't forget the other little things you can do to get really healthy. The more you prepare for your Camino, the more physcially in-shape you are, the more you will enjoy your Camino.
Best of luck and don't let this get you down; just keep pushing and September will be here before you know it.