Preheat oven to 325° F. Grease a loaf pan really well, and line with parchment paper so there is an overhang on both long sides of the pan. If your loaf pan is smaller, line the shorter sides with parchment as well, to prevent the batter from spilling over. (Place on top of a sheet pan, for extra assurance.) In the bowl of a stand mixer, or with a hand mixer, cream butter and sugar until light and fluffy. Add eggs one at a time. Mix well. Add flour, 7 Up, lemon extract, and juice and zest of one lemon (minus two tablespoons), if using. Mix well, but do not overbeat. Bake for 1 hour and 45 minutes (but start checking at 1 hour and 30 minutes to see if a cake tester comes out clean). While the cake is baking, make the glaze. Mix confectionary sugar and lemon or orange juice together, put low heat and keep mixing until fully incorporated. Take off the heat. Once cake has completely cooled, drizzle glaze over the top.