What are the causes of low back pain (lumbar spine area) after a long walk?

Refer to answer: Pain in lower back has three causes, due to long standing. Most common cause is labile disc, facet disease, alone or synovitis secondary to gout, and muscle fatigue. Refer to your physician for further advice.

Because the pain is : Brought on with activity it is unlikely to anything serious like a tumor, fracture, infection or a referred pain from another organ system like a kidney stone . It is musculoskeletal & may be some degenerative changes that may be best addressed with some core exercises like yoga or pilates. Make sure you are exercising regularly, not smoking & not overweight. If symptoms persist, see your pcp.

Spine Pain Options: This chronic pain in the distribution as you suggested is the result of an irritated nerve or facet joints or other injury typically in the spine (low back) which are caused by herniated disks, spinal stenosis or degenerative disc disease, etc requiring further evaluation by a spine specialist and may be candidate for facet injections/radiofrequency ablation and epidural steroid injection.