The other choice is the Hiking from the north, you will travel along St. Helen Road (Highway 30) and then to NW Yeon Avenue.  Finally onto NW Naito Parkway.  At Tom McCall's Waterfront Park on the edge of downtown, you have two choices traveling south:  hike either on the west side of the Willamette River or the east side of the river. 

The map above shows both trails as they merge in Sellwood.

Broughton Beach to I-205 along Marine Drive

This four mile trail can be windy at times, but this might be the most beautiful, convenient and family-friendly section of the loop. It runs right along the Columbia River from the Broughton Beach boat ramp out to I-205.

You can park at the boat ramp or enter the trail behind Pacific Power Boats at Northeast 33rd and Marine Drive. You'll see the entrance under the Northeast 33rd overpass about 20 yards down the street. Get on the trail away from the traffic and make up time as you speed toward Broughton Beach, where you'll cross Marine Drive. Here the trail parallels the Columbia River only feet from the waterline and seemingly miles from busy Marine Drive.

Hikers and riders won't feel rushed because the trail is wide and accommodating. Seasoned riders may enjoy riding on Marine Drive all the way to Troutdale.

Adventuresome riders can opt to ride the I-205 spur over the Glenn Jackson Bridge (it has its own dedicated trail in the middle of the span) into Washington, and be comforted to know the ride back to Oregon is downhill all the way.

There is a short gap between I-205 and Northeast 122nd where hikers and riders must pedal on Marine Drive. But at 122nd, the loop draws riders in again for a secluded trail almost all the way to Blue Lake Park, a distance of another four miles one way.

Things to See  Sailboats, big jets at PDX, Mount Hood vistas.

Source for some of the above:  "The 40-Mile Loop: More than a bike trail, and more than 40 miles" The Oregonian, September 30, 2009.