Yesterday I ran the beach from Seattle (Golden Gardens Bathhouse) to Everett (Pigeon Creek TH) in 4:59. The route stays to the west of the railroad tracks and only leaves the beach for short sections in Edmonds and Mukilteo, and then finishes on a short paved trail in Everett.
The challenge of this route is twofold: limited route existence due to tides and the ever-changing terrain of sand, sharp or slippery rocks, and some boulders. I chose the lowest tide day of the summer and was able to bypass all but 100 meters of boulders when the tide was 2' or lower. (The worst part was just north of Carkeek Park)
Technically my effort was supported because I met a friend while passing through Edmonds and he gave me a donut. I filled up water in Edmonds and Mukilteo.
I do not know if anyone has done this before, I didn't find anything in my research, but it's likely been done before. I'm sure my time could easily be bested by a fitter person. This is a fun adventure that's just in my backyard!
Note: This route crosses tidelands that are private property, which is a legal gray area. However, I did not have any encounters with anyone that were not positive and did not see any No Trespassing signs on the beach.