I do a lot of fire road mixed tours and get single track in there when i can, but its tough to find single track that both goes somewhere and is open to bicycling here in the Northwest- most wilderness areas here are closed to all mechanized travel. and most trail systems here are loops.

but touring on trails, two track and fire roads, both abandonded and still in use? yeahh...... I ride lots of great 2,3,4 day rides right from my place that lead to great fireroading it and trailwork...

if you didn't mind mixing up some pavement with the fire roads and trails, i know you could ride a big loop out of Seattle, crossing the cascades midstate, and up into canada on the dry side of the mountains with less than 35 percent being pavement. There are LOTS of meandering, forest service road routes around Western Washington.

Quality riding too. Screaming fast downhills, then UP, UP, UP. most of the roads are below treeline, its only into the North cascades and the Okanagon you'd get a lot of views like in colorado.

Or, towards the south instead, Seattle across to Yakima on the Iron Horse trail, and back across the historic Natchez Trace wagon route, cross thru Mount Rainier National Park on the way to the DARK DIVIDE and a week of quality trail and FS road riding riding circumnavigating two large volcanoes, Mount Adams and Mount Saint Helens.

Then, you could ride west and across to the Olympic Mountains, to ride a couple of fringe routes on the edges (there are no bike friendly trails or roads that 'biscect' the Olympics, it is too rugged. but there are plenty of long 'skirt the edge' rides out there too.....

there's no summer hut systems that i know of in WA state except some destination huts. you have to be self contained for a couple to four days at a stretch, before a chance at a small town resupply.

(I had to take the panniers off the bike on the left to do the portage over the washout)