Course: Sammamish River Trail from Bothell Landing
Start at the Bothell Landing Park (Map link).   Head over the new bridge and turn left heading south on the Sammamish River Trail for 6.55 miles to Dr Call's bench. 
Just after one mile on the trail you will go down a steep hill, veer right to go over a bridge to continue on the Sammamish River Trail.  (If you end up at the UW Bothell campus then you get bonus mileage.)  You know you are on the correct trail as it will take you under highway 405, just keep going on the trail. 
At about 2.5 miles the trail takes you through the Wilmot Gateway Park where there is a bathroom and water filling station.  Use it if needed and remember this for the way back when you might be thirsty.
Stay on the trail, heading south for a long time.  Just after 6 miles you will see several large fields on your left, 60 Acres Park.  This means you are getting close to the turn around.  You will go under 116th Ave St. and then just a few tenths of a mile further you will see a bench on your left.  This is Dr. Call's Bench.  Turn around here.  Run back to the start. 
If you are doing the half you only do one out and back.  If you are doing the full marathon you get to do the out and back twice.  If you are doing the 50K then you get to go out and back a third time to the bathroom, going all the way to the end of the concrete bulkhead before heading back to the start.
Water and snacks will be available for you at the start/finish and half way point of the marathon.