Hi Elk,
Do you have the Portland Bike There! map? You can get it any LBS, and it's got the whole Portland metro area with routes marked. You could use it to get some ideas about where to ride near your house (bike paths, lanes, roads with lighter or heavier traffic), and then just start scouting.
There's also a free download of the Bike There! map as an overlay for google earth, which I really like. The map continues out into some of the outlying areas of Washington County that aren't included on the paper map.
http://www.metro-region.org/article.cfm?ArticleID=15341
This would be a great time to read a book like Dave Glowacz' Urban Bikers' Tips & Tricks. It's a good guide to riding on streets, and heavy on illustrations for clarity.
Springwater is great, because you ride out & back, so you can make the ride as short or as long as you want. There's also a lovely bike path between the Columbia River & Marine Drive. And the trail out in Vernonia is great for beginners, too.
Gorgeous bike & gorgeous dog! We had a shelty that could have been your dog's brother.
Blueskies