Preheat oven to 350°.
Combine first 8 ingredients; stir well.
Place each duck breast half between 2 sheets of heavy-duty plastic wrap; pound each piece to 1/4-inch thickness using a meat mallet or small heavy skillet. Place duck in a large zip-top plastic bag. Add half of juice mixture; seal and marinate in refrigerator 30 minutes.
Combine cubed bread and oil; toss well. Arrange bread on a jelly-roll pan. Bake at 350° for 20 minutes, stirring once. Set aside.
Remove duck from bag; discard marinade. Heat a large nonstick skillet over medium heat. Coat pan with cooking spray. Add duck; cook 3 minutes on each side or until desired degree of doneness. Remove from pan. Cut duck into 1/4-inch slices.
Combine toasted bread cubes, greens, prunes, and walnuts in a large bowl. Drizzle with remaining half of juice mixture; toss well. Divide greens mixture evenly among 4 plates. Top with duck slices.