Combine agave and yeast with 2 cups warm water in large bowl. Let stand 10 minutes, or until yeast foams on surface. Stir in margarine and salt.
Stir in flour 1 cup at a time until dough forms ball. Turn out dough
onto well-floured work surface, sprinkle generously with flour, and knead 15 minutes, or until dough is smooth and no longer sticks to hands. Knead in walnuts with hands until nuts are distributed throughout dough.
Transfer dough to lightly oiled bowl. Cover with damp towel, and let rise
1 hour in warm place, or until dough doubles in size. Punch down dough, and let rise 1 hour more, or refrigerate overnight, if desired.
Line 2 baking sheets with parchment paper or a silpat, or spray with cooking spray. Turn dough onto floured work surface, and shape into 2 loaves, or place the two loaves in a tin. Cover with damp towel. Let loaves rise 30 minutes, or until doubled in size.
Preheat oven to 400°F. Bake loaves 45 to 50 minutes, or until well-browned on outside. Cool at least 20 minutes on wire rack before slicing.