Rule of thumb is that you shouldn't increase your total weekly mileage by more than 10% from the previous week. Also, it's usually a good idea to stop training about a week before, to give your body a chance to be fully recovered for the event.
You should probably get in a roadie group from a local club so you can learn how to ride pacelines and such (if you don't know how).
If you're racing on the 19th, you've got 3 weeks of training, 1 week of rest. That's not much time.
IMO, don't worry too much about the race on the 19th. If you can finish, that's all that matters. Focus your training on events further out; if you try to train too hard for the first race, you'll end up hurting yourself.