Leif Erickson - At the west end of NW Thurman. Take the St. Helens, Vaughn St exit from I-405, left to NW Thurman, follow to end. 405 in Ptld, take St. Helens rd and then take Vaughn St. exit. Drinking fountain at start of Leif Erickson, bathroom and drinking fountain at Cornell. For long runs you probably want to carry water etc.
Forest Park is probably the best place for long runs in Portland.
The park is approximately 12 miles long by 2 miles wide. In addition there are numerous clearly marked connecting trails. It is relatively easy to run more than 20 miles without going on the same trail twice. Most of the park is heavily forested so the running is in the trees. Located within city limits of Portland and Lake Oswego, between Terwilliger Blvd.