20 MPH on an hour ride solo is a tough tough goal. I approached it once last year (19.6 MPH on a 19 mile course with 1000 ft of climbing and maybe 4-5 red lights), but I was in the best shape I've ever been, late in a year where I logged 7000 miles.

From your description, you can probably do well to work on both increased speed and distance without thinking too hard about a 20 MPH ride. Add some interval training if you really want to increase your speed, but also dedicate yourself to logging those miles. There's no substitute for having a lot of miles in your legs.