Liz,
Congrats on the weight loss that is tremendous!
If you have a chance pick up the April 2010 issue of Bicycling an look on page 35. The American Diabetes Association has published some training guidelines for their upcoming "Tour De Cure" rides this season. The plan they have posted is labeled a "high intensity plan" but don't let that scare you. Simply use it as a guideline for how much time you should be logging on a weekly basis. If you don't like it or don't think it will work for you try bumping your mileage up by at least 10% each week. A couple of weeks prior to your century try back to back long rides (60 - 70) miles.
You shared that if you ride two days in a row you are sore - is this a muscle soreness or is it due to your bike? Did you have you take your bike and sit through a bike fitting (typically take a couple of hours). Is your saddle the proper size? Are you wearing cycling shorts? Having a bike that fits and the proper gear will make each ride a lot more enjoyable.
You are off to a good start, keep riding and have fun! Keep us posted on your progress.