hi Rebecca - that's a great goal that you have. I rode the Harpoon Point-to-Point a few years back which was about 115 miles that year. I didn't follow a plan or do enough training, but completed the ride without any problems.
These days I mostly mountain bike and have done several 50 mile trail races. Definitely find a plan and do the long rides religiously as training, you'll have more fun if you're prepped.
Last year was my first real training year and it helped me drop about 20 pounds and many inches, so the biking should help towards your weight loss goals.