Back pain since more than 5 years.

Hi, am a field engineer (Industrial Service) and required to travel a lot on bike (70 to 100Km in a day). Am having back pain (bearable) since more than 5 years. When I bend forward, the back muscles tend to be stiffer and I feel pain (mild) in the lower region on either sides of the spine above the buttocks. If I stand or walk more than 3 hours...I feel pain in both the legs and in the knee joints (on the outer sides). Recently, the pain developed in the buttocks area and it seems some nerve is getting pressed, the pains goes severe during sleep (only when I try to turn or sleep on sides). Mornings, after some activity the pain goes off and again it reappears in evening. Visited a local doctor, was suggested VitD and Muscle relaxant for a month. The pain seemed to reduce during medication but reappeared soon I stopped the medicines. Also, I feel mild pain in my neck muscles when I stretch my head downwards. Have pain in the spine (Exactly at center of chest area). Plz suggest.