Off the top of my head, in the Portland area:

You can ride most of the circumference of Portland by combining rides on the Vera Katz Eastbank Esplanade, the Springwater Trail, the I-205 MUP and the Marine Drive Bike Path.

Just north of Portland there is Leif Erikson Drive and its 70ish miles of tributary bike and foot trails that run through Forest Park.

Hagg Lake (both road biking and trail biking), Banks-Vernonia Trail, hundreds of miles of farm roads to the south and west of Portland, hundreds more of logging roads to the north...

Keith