If you don't have buttermilk handy, you can substitute as follows: put ¾ tsp of lemon juice into a measuring cup; fill to the ¼ measure with milk. Let stand 2 minutes. This will give your milk the acidic qualities you are looking for with buttermilk and save you a trip to the store.
If you prefer to make this as a loaf of banana bread, use a loaf pan and bake for 1 hour and 10 minutes.