I don't know about meeting your criteria of 12-20 mi. but hiking into Green Lakes is a popular but very nice trail. Look at some maps, and I think you can go about as far as you want, out and back or make a loop of it. Many trails in the Three Sisters area.
As far as bike rentals go, check out these guys
http://www.pinemountainsports.com/There are probably other bike rentals as well.
Food and beverages abound. Deschutes brewery is one of many.
Good luck and have fun.