Preheat oven to 350. Prepare a 13" x 9" pan with cooking spray and flour; set aside. To prepare topping, combine margarine and brown sugar in a skillet. Mix until melted and smooth. Spread to cover in prepared pan. Arrange pineapple slices, cherries, and walnuts over margarine mixture. To prepare batter, combine cake mix, pudding mix, egg whites, water, and oil. Mix until well blended. Pour evenly into prepared pan over top walnuts. Bake for one hour. Cool completely before turning over. Cut into wedges.