Miles for Music not only has a 20k, it also has a 5k and a 1-mile event. Money raised at the event goes to school music programs, and you can run for a registered school to increase their portion of the funds.
As a team championship race, the 20k will give you an early “category 3” 700-point race in the 2016 New Balance Grand Prix.
Make sure your USATF membership is current, and if you’re not already running on a team, why not join a club and see if they have a USATF team. Even if you consider yourself a slow runner, there’s a club out there waiting to cheer you on.