Your leg pain could be caused by one of a few different reasons. If you are diabetic, it could be related to your circulation. If you have not exercised in a while, it could be that you are getting a lactic acid production, causing your pain. If you have joint misalignments, there could be pressure one one or more of the lumbar nerves, leading to what is known as Sciatica.
Be sure that you are staying hydrated, and be sure to stretch before and after walking.
If your pains persist, seek Chiropractic care first, drugs second, and surgery as a last resort!