We parked at Marymoor Park, parking is $1 for the day, the lot by the baseball fields will put you closest to the start of the trail on the west end of the park. We ran North on the Sammamish River Trail from Marymoor Park to the "Burke-Gilman trail" sign near Blyth Park. Then we turned around and ran south back to the "Sammamish River Trail" sign at the entrance to Marymoor Park.
Melissa and I ran together carrying all our own water and nutrition, with no need to resupply. The weather was warm and sunny and the trail was busy. Lots of cyclists, other runners, families, and walkers with their adorable dogs, and one adventure cat. Most of the trail is exposed with minimal shade. There are facilities for water resupply and restrooms multiple places along the trail.
For a flat paved urban trail it is quite nice, scenic and well taken care of. Definitely an easy route for a speedy time. Just look out for fast e-bikes and wild geese.