Dissolve yeast and honey in warm water in a large bowl; let stand 5 minutes. Stir in yogurt and oil. Lightly spoon flours into dry measuring cups; level with a knife. Add 2 1/2 cups all-purpose flour, whole-wheat flour, walnuts, and salt to yeast mixture; stir until blended. Turn dough out onto a lightly floured surface. Knead until smooth and elastic (about 10 minutes); add enough of remaining flour, 1 tablespoon at a time, to prevent dough from sticking to hands (dough will feel tacky).
Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, 1 hour or until doubled in size. Punch dough down; let rest 5 minutes. Roll into a 14 x 7-inch rectangle on a floured surface. Roll up rectangle tightly, starting with a short edge, pressing firmly to eliminate air pockets; pinch seam and ends to seal. Place roll, seam side down, in a 9 x 5-inch loaf pan coated with cooking spray. Cover and let rise 30 minutes or until doubled in size.
Preheat oven to 350°.
Uncover dough. Bake at 350° for 45 minutes or until loaf is browned on bottom and sounds hollow when tapped. Remove from pan; cool on a wire rack.