Naturally, there are going to be times when you want to have the map before you go and explore. Fortunately you can trace the route in TOPO! and then export that data back to the GPS device and follow along on the screen. The Edge 305 isn't a true mapping GPS as it will not display an actual topographical map on the screen, but it will show you a graphical display of the route you're supposed to follow which is good enough for times when you don't know if you need to turn right or left.
Judging by the maps (and rumors I've heard), it seems possible to follow the route I did today and turn onto a grassed-over 4WD trail that follows the powerlines around Rattlesnake Mountain and up to Tiger Mountain. Tiger Mountain is only a few miles from my house, but the direct way there is on a shoulder-less highway and not something I wish to ever ride on. If I can make a lengthy loop and work my way from the Snoqualmie Valley Trail all the way up to the Radio Towers atop Tiger Mountain, I'd be very happy. Not to mention it would be a killer workout. Sounds like the perfect opportunity to trace a route in TOPO!, export it to the Garmin, and see how well I can navigate from it.
Who says toys are for kids?