You must do this a minimum of 3 times per week to maintain your present level of fitness. Unfortunately what passes for aerobic exercise today is in reality its antagonist, anerobic exercise. Anerobic exercise utilizes carbohydrates for energy. The body doesn't store much carbohydrate in the muscles to be utilized as energy. It is depleted rather quickly during anerobic energy use.
Lets spend some more time discussing the differences in aerobic and anerobic exercise.
Aerobic exercises utilizes fat burning factories, called mitochondria, and requires oxygen. Anerobic exercise utilizes sugar-burning enzymes found in most cells and doesn't require oxygen. Aerobic exercise requires Iron as its primary nutrient, while anerobic require certain B vitamins, especially pantothenic acid. Aerobic exercise is performed during a certain heart beat range, while anerobic exercise is either above or below that. During aerobic exercise you will be able to carry on a normal conversation with someone, while anerobic exercise tends to leave you breathless.