Based on the symptoms you describe, I would say you have nerve root compression in your spine. Possibilities are a herniated disc or stenosis (narrowing of the spinal canal, often with age). If you haven't already visited your general practice doctor, now is the time to do that.
He or she will probably start you with some pain meRAB, NSAID's, a muscle relaxer, and physical therapy. You may also get ESI's (anti-inflammatory injections). This conservative treatment works for the vast majority of people, and you will most likely be in this situation.
If this regime doesn't work after several weeks or months, your doctor may order an MRI to determine just what is going on. If there are problems that surgery may help you will be referred to a surgeon.
How long have you been experiencing this pain and where down your leg does the pain go?