The only flat bike ride in Seattle follows the Duwamish river to Tukwila and beyond. The route can be a little tricky getting through a few areas, but we are leading a ride this Saturday at noon to go from West Seattle Cycle U to Tukwila and back which is 22 miles and we will be going a 12-14mph pace and regrouping as needed.
This route actually links a few great and less used bike paths together going South and when linked to the Burke Gilman trail North after Magnolia (which can be too busy when the weather gets this nice, but it is flat) can give you more flat miles than most people want to ride in a day.
The ride we are leading on Saturday is the basis for a longer flat ride South. From Cycle U in West Seattle to Sumner it is a pretty flat 33 miles if you want to keep going past Tukwila, round trip would be roughly 66 miles. Here is a link with cue sheet and map that shows it as part of a longer ride going into Browns Point, but the route is from mile marker 35-49 but like most rides it is better to have someone show you the first time to save wrong turns and poor roads- Link to cue sheet and Map With this weather it is time to get out and ride!