I guess this is a "I went for a ride and this is what you missed" kinda thread.
Floman and myself did the North part of the Cascade Loop Winthrop yesterday. Here's our summary.
Getting to Sedro Woolley via Highway #9 through the Sumas border is scenic, slow, very smooth, with the odd 90 degree turn where you can hang it out a bit.
Once in Sedro Woolley, you have two options. Head East on Highway #20 or West for about a mile to the lights where you can turn left/south to hook up with Highway #9 again heading South to Clear Lake (8 miles).
In Clear Lake look for the Skagit County Road immediately on your right (yes, right) immediately after the bridge. On a map this is highway #538. That road loops under the bridge and heads out East parralel to the #20 and meets up in Concrete.
Beautiful scenery all along the river, but kinda bumpy. They were oiling it the that day (gross), which was ironic as at the same place the sign showed that the next few miles were all twisties (bummer). I wouldn't bother riding this road again unless it was paved.
From Concrete we ended up falling in behind a state trooper (85kmh) for what seemed like forever. Concrete is notorious for state troopers being on the outlook for speeders. Everyone says "Don't speed anywhere near Concrete". After 20 minutes crawling behind this trooper through beautifull billiard table smooth twisties we pulled over at the Rockport State park (where there is a big train is on display). Hung out in the shade for about 45 minutes.
Got going again and ripped it up all the way to Winthrop, never once seeing a cop (this is between 11:00am and 2:00pm). On the return, same thing. Never once saw a cop and my radar detector never beeped once. Make sure you gas up in Conrete as there is no gas between there and Winthrop.
Even though #9 going down to Sedro Woolley is slower in speed than taking the I-5, I think I still prefer it. The I-5 was just too straight, too fast, too tiring, and I was getting all angry at bad drivers. Highways suck!. Plus I think it's a longer way back.
When I do this trip again I'll only go as far as the Diablo or Ross Lake Dams (pic is at Diablo Dam) which is half way to Winthrop, then turn around and come back. Ain't much to ride past there.
Total trip time (less the time we wasted with me getting lost trying to find Scagit County Road) would have been about 7 hours with a break for lunch. If you choose to just go to the dams at the summitt of the North Cascades on highway #20 and back, then it shoudln't take much more than 5 hours.
Have fun! The quality of pavement, twisties, and scenery is well worth it.