The Trails Club has weekly walks starting in Northwest Portland on the Wednesday morning at 9:30 as well as other activities.
The Cascade Prime Timers have weekly two hour Rambles on Mondays at mornings at 10 AM and more ambitious hikes on Fridays as well as other activities.
The Mazamas have two hour Street Rambles about Portland every Tuesday and Thursday evening leaving at 6pm from the REI garage (NW 15th and Johnson in the Pearl district) as well as other activities.
WalkOregon.org lists Volkswalks in Oregon. Volkswalks events are usually laid out as 5k or 10k routes where you can start the walk at anytime between 9am and 1pm and walk at your own pace. The site offers information map-guided and hosted walks throughout the state, as well as other ways to find out about walking and hiking opportunities.
The Ten Toe Express, part of SmartTrips Central City sponsored by the city of Portland, are guided walks through parts of Portland.