Preheat the oven to 350°F. Grease a 9" x 5" loaf pan., In a large bowl, whisk together the flours, sugar, baking powder, baking soda, and salt., In a separate bowl, mix together the starter, buttermilk, melted butter, eggs, maple syrup, and maple flavor. Add the wet ingredients to the dry, stirring just until the mixture is evenly combined. Stir in the walnuts., Spoon the batter into the prepared pan. Bake until the top of the loaf is golden brown and a paring knife inserted in the center comes out clean, 65 to 70 minutes. Tent the bread with foil after 45 minutes if it's browning too quickly., Remove the bread from the oven and cool in the pan on a rack for 15 minutes before turning it out of the pan. Return to the rack and cool completely before slicing., Store the bread, tightly wrapped in plastic, at room temperature for several days; freeze for longer storage.