Preheat oven to 350 degrees F. Grease and flour two 8x4 or two 9x5 inch loaf pan. For a larger loaf, just use one 9x5 inch loaf pan.
In a large bowl, thoroughly mix together the flour, sugar, baking powder and salt. Add the toasted walnuts and stir well.
In a second bowl, mix together the egg, marmalade, orange juice and oil. Add the liquid ingredients to the dry ingredients and stir gently until mixed.
Pour the batter into the prepared pans and bake on center rack for 50 minutes to 1 hour. My small loaves took 50 minutes, so I imagine one large would take 1 hour.