Welcome to the world of cycling!
Weight comes off more easily when you make a lifestyle change, and then forget about your weight for a while. Realistic weight loss should be round the 1-2lbs per week, and can vary according to time of day, what sort of week you have had, hydration, etc. Therefore checking week-by-week will result in worrying, despondency, misplaced joy, and too much time spent thinking about what to eat and what to avoid. Just upping your exercise and cutting out chocolate bars will have a significant influence over a long period.
Next problem is how to deal with your next weight plateau. This means further cutting down on food, or making sure that an 18.5 mile ride is what you do on weeks when you are feeling tired and not up for it.