Good for you! A lot of overweight people are too intimidated to go to the gym let alone get out on the road and ride.
As far as doing the rides you mentioned, I'd say just get out there and put in some TITS (time in the saddle). You can do this.
I have some experience in losing weight, about thirty pounds. I went to Weight Watchers, learned how to eat healthy foods, swam and lifted weights. I got certified as a personal trainer then went back to college and got an AA in health and Phys. Ed. (I did that mainly for myself, I have a chronic liver disease)If you belong to a gym, start taking spin classes. The intervals are a great way to lose weight and improve your aerobic capacity. It's a great way to train during winter as well.
Don't even think of it as a hundred pounds, break it down into small increments. Don't be a slave to the scale, either. Once a week should be fine. Let your clothes be your gauge.
If your schedule allows, you might want to look into Weight Watchers. You might find other members there who can relate and help you better than I have (or haven't).