Dear patient there are two reasons for your back 1. facetal degeneration and hypertrophy suggestive of instability 2. disc degeneration and spurring with arthritic changes may be related to age. The reason for pain in limbs is canal stenosis with nerve compression and ischaemia resulting in neurological claudication. Facet joints are on both sides of vertebra and holds both vertebra together with intervertebral disc. Degenerative changes in disc leads to loss in their height which strains facet joints. In turn facet joints reacts by hypertrophy and arthritis trying for natural fusion. All this process leads to back pain. Nerve roots pass through narrow canal known as spinal canal. Narrowing of canal due to bony spurs and ligamentum flavam hypertrophy leads to canal stenosis. This impedes blood supply of nerve roots and pain in lower limbs while you walk. you need to consult spine surgeon to discuss various treatment options like canal decompression or fusion. all the best.