In a small bowl dissolve yeast in the 1/2 cup warm water; set aside. In a large bowl combine all-purpose flour, whole wheat flour, mashed potatoes, cottage cheese, the 1/4 cup water, the oil, salt, dillweed, thyme, and, if desired, celery seeds, stirring until well mixed. Add the yeast mixture; stir until combined.
Turn dough out onto a lightly floured surface. Knead until dough is moderately soft, smooth, and elastic (3 to 5 minutes). Shape dough into a ball. Place dough in a lightly greased bowl, turning once to grease the surface of the dough. Cover; let rise in a warm place until double in size (45 to 60 minutes).
Punch dough down. Turn out onto a lightly floured surface. Cover and let rest for 10 minutes. Lightly grease a 1-1/2-quart casserole or a 9x5x3-inch loaf pan.
Shape dough into a round or rectangular loaf. Place round loaf in prepared casserole or place rectangular loaf in prepared loaf pan. Cover and let rise in a warm place until nearly double in size (30 to 40 minutes). Preheat oven to 350 degrees F.
Bake for 35 to 40 minutes or until bread sounds hollow when tapped on the top. Immediately remove from casserole or pan. Cool on a wire rack.