Our Saturday morning "B" ride historically averages a true 15.5-17.5 mph over reasonably hilly terrain (not counting stoppage time). Subsequently it's not unheard of to maintain 21-24mph along reasonably long stretches of flat road.
Give it ago again next week. But, this time, ride it smarter. No chasing after the fast guys. Attempt to maintain the second or third wheel position. Don't volunteer to go to the front and pull, unless people naturally rotate off the front of you. In which case, take a pull, but, keep it reasonably short. Don't linger at the back. The gaps and excellerations are exagerated back there.
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Birth Certificate, Passport, Marriage License Driver's License and Residency Permit all say I'm a Fred. I guess there's no denying it.