I am pretty sure you can automatically do the export with
www.mapmyride.com. Create your route in there, then export as a GPX file. You then should be able to import the file using the Garmin software to the GPS. I am at work which does not allow Java/javascript otherwise I'd go on the site and figure out hwo to do it.
The only part I am not sure about is the ability of the Edge 305 to import the data from a GPX file, since I do not own one. It should do it.