Preheat oven to 350°F (176°C) and lightly grease a 9" x 5" loaf pan.
Grate the zucchini using the large holes of a cheese grater. Place it in a colander and squeeze out excess moisture. Leave the zucchini in the colander while you mix the remaining ingredients.
In a large bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon and chopped walnuts. Set aside.
In a separate bowl, mix the light brown sugar, eggs, sourdough starter discard, vanilla extract and oil with a spatula until smooth. Pour the dry ingredients into the wet ingredients and mix until just combined. Stir in the grated zucchini.
Pour the batter into the greased baking pan and bake for 55-60 minutes, or until a toothpick inserted in the center comes out clean. (See notes)
Let the bread cool for 10-15 minutes before transferring it to a cooling rack. Slice and serve.