The WA Back-country Discovery Route runs from Stevenson WA north to the Canadian border on (mostly) gravel forest service roads.
Good documentation of the route is on arcgis.com.
I've analyzed their maps and re-created the route using Google Maps.
It looks like the route can be done in a comfortable 6 days with hotels at each nightly stop. Then you can get back in one 8 hour, 400 mile day.
The symbolic beginning of the route involves crossing the "Bridge of the Gods" from Oregon into Washington, just west of Stevenson, WA. Gas, groceries, restaurants and hotels are available in Stevenson. The route heads northeast over Triangle Pass and Trail Peak on gravel and dirt roads through the Gifford Pinchot National Forest.
On a clear day, riders will be treated to views of three volcanoes: Mt. Hood, Mt. Adams and the infamous Mt. St. Helens; which lost 1,300 feet of elevation in a massive eruption on May 18th, 1980.
A short detour from the route to visit the Guler Ice Caves is worth the time. Created by a lava flow over 20,000 years ago, this lava tube preserves ice formations year round. The ice caves are West of the town of Trout Lake, near Peterson Prairie Campground.
The route continues over Trout Lake Creek, Grand Meadows, Takh-Takh Meadows, Midway Meadows and then down to Hwy 12 arriving at Packwood where gas, hotels, restaurants and groceries are available.
Hwy 12 heading east from Packwood crosses over White Pass and skirts Rimrock Lake before turning north onto gravel roads which lead up to Bethel Ridge. Stunning mountain views including Mt. Rainier and Mt. Adams can be seen from a basalt ridge that drops off sharply to the Tieton River Valley below.
After a rugged section of rutted roads along the ridge, a long descent brings you to Hwy 410 near the small town of Nile. After a short section of pavement, a dirt road heads up over Cleman Mountain, passing through the Oak Creek Wildlife Area and then down Evans Canyon. A short paved section on Wenas Road North leads to a steep dirt road that winds up and over the rocky and treeless Umtanum Ridge.
After descending the historic Old Durr Road, there is a small water crossing at Umtanum Creek and a section of asphalt running into Ellensburg which is a large town with full services.
Paved roads head north from Ellensburg toward Table Mountain and Lion Rock Lookout in Wenatchee National Forest. High elevation and twisty dirt roads with views of the Central Cascade Mountains make this a very enjoyable section. From Lion Rock to Beehive Reservoir the route is stunning, rugged and remote. The path at 6,000 feet along Table Mountain provides views to the west. Elk can often be seen in the beautiful Haney Meadow.
From there the road traverses a ridgeline with spectacular views of Devil's Gulch and the Columbia River to the North. The route passes nearby Beehive Reservoir before descending the Mission Creek Valley to the town of Cashmere which has gas, groceries, restaurants and hotels.
Heading north up Nahahum (pronounced Nah-Hum) Canyon from Cashmere brings riders to a 360-degree view atop Chumstick Peak at 5,810 feet elevation. The route winds up and over Roaring Ridge in the Entiat Mountains before descending steep switchbacks to the one-store town of Ardenvoir which has a small cafe, gas and basic rations.
The next 30 miles have a gain of over 4,000 feet as riders climb McKenzie Ridge and the Chelan Mountains. Views of Lake Chelan can be seen as travelers pass through the Slide Ridge area on the descent to 25 Mile Creek State Park. A pavement road follows the Southern Lake Chelan shoreline east to the full-service town of Chelan.
The route ascends north from Lake Chelan past the Echo Valley Ski Area toward Cooper Mountain, a 5,867 foot summit with stunning views. The route then passes Black Canyon Creek and skirts Fox Peak before descending the Gold Creek Valley to the Methow River and Hwy 153.
Heading north on Hwy 153 riders will pass the one-store town and gas pump in Carlton before heading east on Benson Creek Road. The path heads up past Thrapp Mountain and Woody Mountain before dropping down into the valley to Hwy 20.
After a short highway section riders are back on dirt heading north on scenic Loup Loup Canyon Road in the Loup Loup State Forest. The steep Ruby Grade descends toward a site where the mining town of Ruby once existed. A sign at the site describes a bustling mining community that was the center of commerce in the Okanogan area until the market for silver collapsed in 1892.
Gravel and paved roads take riders to a small town of Conconully, which has a motel, store, and a gas pump. Additionally, there are lakes and a state park in the area.
The route heads north from Conconully and pavement eventually turns to dirt again as riders reach Salmon Meadows in the Okanogan National Forest. Riders soon enjoy views as they climb up and over Lone Frank Pass (the highest point on the WABDR).
After a short distance the route enters Loomis State Forest and heads up the ominously named "Skull and Crossbones Road" where an old homestead cabin and corral can be seen before heading down to North Fork Toats Coulee Creek. There is a geocache near the viewpoint above Cold Springs Campground up Ninemile Creek Road (see map for coordinates).
The route then winds down steep switchbacks into a spectacular valley just south of Palmer Lake in the Sinlahekin Valley. With the dirt and gravel behind you, pavement leads you north past Palmer Lake through the former town of Nighthawk along the Similkameen River to the Nighthawk border crossing from the US into Canada.
The day begins with a 1:30 hrs run of 76 miles to the Grand Coulee Dam visitor center.
2:20 hrs and 127 miles to the Priest Rapids Dam in the Hanford Reach National Monument.
2:15 hrs and 115 miles to the Canyon Market in Klickitat for lunch.
1:45 hrs and 82 miles back to the Chalet via highway 14 on the north shore of the Columbia River.
Or here's a more interesting route back, perhaps with a stay in Ellensburg to break it up