oh, you could start at marymoor and ride to The Tolt, to the SVT, to the Iron Horse to the pass, and return iron horse to the svt to northbend and then via snoq falls to the the railroad connectors to issaquah and the eastside sammammish to marymoor makes for a nice big ride with an awfully high proportion of gravel.

i do a gravel mixed surface century trail ride out of seattle that goes Burke-Gilman to the Tolt, to the SVT, then to Snoqualmie Falls and the trestle over the river, then back down to snoqualmie, then along the preston connectors down to Issaquah to the eastlake Sammammish trail and return to seattle.

90 percent off road, maybe 60 percent gravel? Starting in Marymoor and including the Iron horse option and maybe a little wandering towards black diamond could get you to a 200k i suspect with a lot of out there. i think the iron horse alone is over 60k round trip from rattlesnake lane to the tunnel.