At the very beginning for a rider you have to be very careful about adding miles. You are beyond that and your body is used to riding. In addition to the other posters suggestions I offer the following as I have a 100 miler scheduled in a month with my previous long being 65 (or so) in July:

- Work on making use of all parts of your spin and don't just mash your pedals. I used to do that and still work on improving.
- You mentioned doing a 40 mile ride. How did you feel after that? I suggest riding in one direction 25 miles so you have to ride back 25 miles. Stop every 15 miles or so and make sure you eat a little something. Practice your long distance nutrition. This will help you ride longer NOW and get you ready for what you have to do on your 100 miler. By going 25 miles in one direction you are forcing yourself out of your comfort zone.
- Don't turn it into a chore. Sure you may have to force yourself out here and there to get to your goal but don't do it too much else you will dread getting on the bike.