Starfish, one of my favorite long gradual climbs is the ride I do starting from Enumclaw on route 410. I start from either the bike shop or National Park Info Center, or the Enumclaw High School and ride to Greenwater, distance is 20 +/- miles depending on where you start. Out and back makes it a 40 mile ride. You can continue past Greenwater and enter Mount Rainier National Park and get a longer ride in. If you go all the way to Cayuse you will get a 42 mile ride (one way). Out and back 84 miles. So a total of 42 miles of gradual climbing and then a great downhill coming back. If you want to add steeper climbs to the gradual ones, take a right hand turn towards Sunrise Visitor Center about 15 miles inside the park. There will be signs. It's a very steep and winding road with little shoulder. Or bike up to Crystal (another steep climb). You can make the ride as long and steep as you like. Some people just like the steep climb and drive to the White River entrance and park just past the information center. Then they cycle up to Sunrise and then back down to the White River Visitor Center. I live in Bonney Lake and Enumclaw is only 11 miles from my door step. I usually do it once or twice during the summer. I may have to do it again this summer. I always did enjoy the Enumclaw to Greenwater ride (evenly graded but you know you're climbing). I usually go past Greenwater and do a 25, 30, or 35 mile ride out and then turn around and come back.
I can't think of anything longer near me. I do have a lot of favorite rides in the Black Diamond/Enumclaw/Maple Valley area. But they are mostly roller coaster rides. A lot of up and down. The longest one is the Flaming Geyser to Black Diamond loop. You climb the Auburn Black Diamond Road. A good evenly graded 10 mile climb. But like I said it's mostly roller coaster hills, 2-4 miles long.
Of course you can always do Blewett Pass (if you feel like driving to Cle Elum or Leavenworth). That's an evenly graded climb. Except for the last 5 miles that is! It gets pretty steep just before the crest. I did the Courage Classic in 2003 and I absolutely loved that climb! The downhill part was a blast!
Sue
Forgot to mention, once you enter the national park the roads are narrow. No shoulders to speak of so I usually turn around at the park entrance for the gradual climbs.