It seems that the common rule I hear is to ride often (and rest often), to vary the types of rides (some slow and long, some fast and short) and to increase the distance of your "long" ride each week by 10%
If you ride 10 miles for your long ride now, and increases 10% each week, you will be riding over 100 miles in 26 weeks... and that is just a guideline. You should be able to bump it up a little bit faster.
Just remember that your body will need rest mixed in along with the increased activity.
By the way, I am at about your weight, and plan to ride 100 miles in Philadelphia for the LiveStrong Challenge. I don't know your age, but I am 51, and have in the past few years ridden rides of up to 68 miles (although I weighed a little less for them, and will if I hope to make the complete LiveStrong ride).