There is no easy way that I know of, but here is how I do mine: Map your route using a web site like mapmyride.com, then save and export the route to a kml file. Open the kml file in Training Gym Pro. Go to Data Transmission, Track Points, then select Upload to device. Once the file is uploaded, then go to menu on your 580P, select Activity, Activity Info, Training Info, choose the file you just uploaded, select Track Back, select either Forward or Backward and you are good to go. It is not as easy as the Garmin 500 but you probably didn't pay the Garmin price either.
I used the track back function to navigate a 68 miles ride yesterday, I only had to make a U turn twice. So I am pretty happy with it. I will be happier if the 580p would continue to navigate after I deviated slightly from the uploaded route.