23 year old male. 5 11 160LBS. I used to bike ride a lot when I worked as a bike messenger when I was 17-18 years old. I rode a giant OCR 3 back then. Since then, I've lost interest and riding but am now motivating and want to get back into it. I'd like to spend at least 15 hours a week riding. My budged for a bike is 300-600. What would be the best bike in this price range? I know this isn't much, would I be better off buying a used road bike or a new one? What are the best training techniques when you're just getting back into riding? What is a good goal for miles ridden a week, etc?

Thanks for all your help!

Seymour