Preheat oven to 325 degrees. Spray large loaf pan lightly with cooking spray on all sides.
In a small bowl, mash ripe bananas. Set aside.
In large bowl, mix together stick of softened margarine, sugar, eggs (mix in one at a time) and vanilla. In the same bowl add flour, baking soda, salt, and mix well. Slowly mix in milk, bananas, and walnuts.
Put everything into loaf pan and bake for 1 hr, 20 minutes. After one hour, put knife in center and if it comes out clean it is done. If not, check 5 minutes later, but try not to overbake which could cause dryness. Cool in pan 10 minutes before removing.