You don't need to do 100 mile rides. You wouldn't race even half that distance at this point. And average speed matters very little in actual races. Trying to ride a solo 100 miles as fast at you can is very unspecific for the actual racing you'd do. Not that it can't be fun or good general training or a chance to explore or any of a number of other reasons, but for race training it's one of the last things I'd recommend doing or do. I can count on one finger the number of solo 100 mile rides I've done in the last 5 years, and in that same time period I've actually raced a few 100+ mile races.
Far, far, far more important is learning how to ride safely and efficiently in a group. If you want to race, getting plugged in to the faster/harder/better group ride scene is the most important first step (to being successful).
Especially in your locale and at your weight, you're going to have to learn how to be a ghost in the field, meaning you need to learn how to be very good at drafting, moving around in a group, and putting yourself into good positions for the finish.