Where do I start with cycling? I come from a running background and I recently started cycling due to an injury. My longest rides as of now are 15 miles in 1.5 hours. I hope to cycle a century in a few months or so. But I've only been using a random bike that I've inherited.

What bike should I buy? My budget is 600 - 800$ and I only bike on road and flat surfaces. I have no idea on what brands or what parts are good. Should I get fitted at a bike shop? Should I build my own one?

Also, is overtraining a thing in biking? In the running world, injuries are pretty easy to get if you increase mileage too quickly but I'm not sure if this applies to biking aswell. Is there some sort of program that builds you up to ride for longer similar to Couch to 5K (C25K)?