There's no way to really know without a doctor's exam. Aching legs can be caused by a number of different things. A couple of things that may cause aching legs are vitamin/nutrient deficiencies and circulation problems.
Although several different vitamin/nutrient deficiencies can result in leg aches, two of the most common are calcium and potassium. Dairy foods and leafy green vegetables are the excellent sources of calcium. And here's a link to a list of foods high in potassium: http://www.vaughns-1-pagers.com/food/potassium-foods.htm
If you smoke, have a history of heart disease, or anything else that might result in circulation problems, you should definitely see your doctor to see if there is any link to your aching legs.