What Causes Severe Pain In Knees And Legs?

I have serious leg and knee pain most days. Some days are so difficult to get thru. I have had this pain for years. RA has been ruled out and I was not very athletic growing up. I m only 51 but this pain is making me feel 81. I do have some arthritis in my knees but doesn t explain this antagonizing ache. What can this be?