So where is Portland, eh? I guess we need to establish that before we start marking off the "12 miles north."
To me, and most locals I know, Portland starts at the Columbia River. When you cross into Oregon on I-5, you feel like you're entering Portland as well. PDX is off to your left, and you can see the planes on approach.
Hence, 12 miles north of Portland in the vernacular puts you into the Orchards/northern Vancouver area.
However, the drive into the heart of Portland takes forever! It's at least 10-12 miles before you start seeing the downtown buildings and stadium.