You should check out joining the Seattle Randos. All sorts of 200 and 300k rides, a fun group to ride with, and riders of all speed levels out there so I'm sure you could find someone to ride with (if you wanted to) or just hang solo if you're more that kind of rider.
Bonus: RUSA membership is only $20/yr, SiR is only $10. So for $30 you can ride as many permanents as you want for the cost of only the postage to turn in your time card, and most brevets are only $10 or $15 for 200 and 300k. I think the 600k level goes up to about $50 or $75 sometimes. My last 400k reg fee was only $25.
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"I feel like my world was classier before I found cyclocross."
- Mandi M.