At the pace you're riding, and looking at 200k, you might do well to check out the Seattle International Randonneurs. While there are a few speed demons in the group, there are more people that you'd find yourself riding with than getting dropped by. Especially if you're interested in doing STP as a one-day in the future, the Randos are a great bunch.
Rando isn't about how fast you finish, it's about self sufficiency and persistence. I averaged under 13mph overall on most of my longer rides. 300k in 15h 03m (overall average 12.6), 400k in 24h (overall overage 10.4). The thing is that your overall average includes your stoppage time, so on a long-arsed ride like 400k (250 miles) you're taking into account all your stops for food and water resupply, bathroom breaks, lunch/dinner stop, mechanicals, etc.
It's a self-supported discipline, so there's no SAG wagon and no outside help allowed between the registered control stops. You must follow the route as it is written on the cue sheet, and if you miss a turn you don't get to "find your way" to the next turn, you go back to where you made the mistake and chalk it up as bonus miles. Some people ride events as groups, others prefer to roll solo; it's all up to you. The only rules are 1) Follow the course exactly, and 2) Finish within the time limits. Each control station has an open and close time, and every event has a time limit based on the distance:
100k - 6h 30m
200k - 13h 30m
300k - 20h
400k - 27h
600k - 40h
1000k - 75h
1200k - 90h
1400k - 110h
As you can see, it's not about the speed. The overall average works out to under 10mph. The real challenge is that you bring or buy everything you need along the way. There's no REI sponsored rest area with free food and electrolyte drinks every 20 miles like on a big sponsored century ride. However, on most of the 200k and over events there will be a control station manned by a few of the randos who are handing out some awesome grub (which is a relative measure: Cup 'o Soup is awesome at midnight in a chilly downpour with another hilly 100k to go.)
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"I feel like my world was classier before I found cyclocross."
- Mandi M.