1. Preheat oven to 350°.
2. To prepare bread, lightly spoon flour into dry measuring cups; level with a knife. Combine flour and next 3 ingredients (through baking soda) in a large bowl, stirring with a whisk; make a well in center of mixture. Combine granulated sugar and next 4 ingredients (through 1 tablespoon juice). Add sugar mixture to flour mixture, stirring just until moist. Fold in zucchini and walnuts. Divide batter between 2 (8 x 4–inch) loaf pans coated with cooking spray. Bake at 350° for 50 minutes or until a wooden pick inserted in center comes out clean. Cool 10 minutes in pans on a wire rack; remove from pans.
3. To prepare glaze, combine powdered sugar and 2 tablespoons juice, stirring with a whisk. Drizzle evenly over warm loaves. Cool completely on wire rack.