The most common cause of aching legs is poor blood circulation in the leg arteries. This produces an aching, tired, and sometimes burning pain in the legs. A secondary cause of leg pain is strain and stretching of the calf muscles as a result of poor foot function.
As the feet roll inwards (over-pronation) the legs rotate internally, placing stress on the muscles at the back the legs. Standing for long periods of time will make this condition worse.
Wearing high heeled shoes also contributes to tired, aching legs.