Get the Bike Portland map at Powells or a local bike shop. I'd probably take Kerr Pkwy/Capital over to Garden Home, go over to Scholls Ferry via a side street, then take that to Burnside/Barnes Rd. You'd be looking at about 12 miles, with some nice hill work. If you do a couple of hours of hill work each work day year round you'll end up in very good shape.
I used to live in Mt. Park and there are some fun ways to get to Capital Hwy up past PCC Sylvania.