Dissolve yeast and sugar in warm water in a large bowl; let stand 5 minutes. Lightly spoon flours into dry measuring cups; level with a knife. Add 3 cups all-purpose flour, whole wheat flour, milk, oil, and salt to yeast mixture; stir with a wooden spoon until smooth. Turn dough out onto a floured surface. Knead until smooth and elastic (about 10 minutes); add enough of remaining flour, 1 tablespoon at a time, to prevent the 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. (Press two fingers into dough. If the indentation remains, the dough has risen enough.) Punch dough down; cover and let rest 5 minutes. Divide the dough in half.
Working with one portion at a time (cover remaining dough to keep from drying), shape each portion into a 6-inch oval on a floured surface. Cover with a slightly damp towel; let rise 20 minutes (dough will not double in size).
Working with one portion at a time, stretch each portion into a 15-inch-long loaf (loaf will be flat); place on a baking sheet coated with cooking spray and sprinkled with cornmeal. Cover with a slightly damp towel; let rise 30 minutes (dough will not double in size). Lightly dust each loaf with flour.
Preheat oven to 425°.
Bake at 425° for 25 minutes or until loaves are browned on bottom and sound hollow when tapped. Remove from pan; cool on wire racks.