I’ll update this with miles, etc.
1. Lake Washington. From my house: down to Marymoor (early, before people get up — even then, you’ll be dodging walking clubs/jogging threesomes), N on Burke-Gilman around the top of the lake*, then follow the UW-Seward instructions to get to Lake Washington Blvd, take that to Seward, Seward to Jean Coulon, then take the trail up along the W side of Lake Washington that kicks you out at the I-90 Coal Creek underpass. Added bonus: throw in some Mercer Island loops for extended hill work to hit your distance/hill goals.
* I love the stretch through Woodinville/Bothell early in the morning, when no one’s there and it’s still a little bit cold as you whip through the turns. The poor trail condition (roots! roots! roots!) down to and through the UW is hard. Burke Gilman through Redmond becomes more and more dangerous as morons fill it, to the point where you may as well sit up and soft-pedal it through.
2. Duvall-Snohomish-Monroe-Duvall. I used to ride this all the time when I lived out there. Head out from Duvall and follow the Flying Wheels route. Pretty easy on the hills. Lots of easy rural roads. Only problem is you’re exposed with short shoulders on some curves, which would be scary if there was more traffic. For added fun, you can throw in the loop down to Carnation.
3. Not-so-Chilly Hilly. 33m, ~2600 feet of climbing. Take the ferry out to Bainbridge, do the Chilly Hilly route. Repeat as required to reach your distance goal, or until you die. Beautiful. Also painful as all hell, especially if your friend is (say) 25% lighter than you are and likes to kick your ass up long gradual climbs. Seems like it’s hard to get food/water during the ride.
Another way of increasing miles on the Lake Washington loop is to loop around Seward Park a couple times before continuing south.
To increase hill intensity on that ride, instead of sticking to the lake, go up and over Capitol Hill – leave the BGT at Husky Stadium, go across the Montlake Bridge, up 24th, over to either Madison or Yesler, and down to the lake from there. The climb up 24th SUCKS, but the ride down Madison is fun.
Here’s another great mega-miler. Leave your cousin’s house in Yakima, ride through the gorgeous yet poorly-shouldered (fortunately lightly-trafficked and also well-troopered on weekends) Yakima River Canyon to Ellensburg. Get on 97 northwest of the ‘burg, turn around when you get to the summit of Blewett Pass. What was that, Derek, 130 miles or so? Great prep for riding STP in a day.