You couldn't keep up on a charity ride with a normal average of 15 mph? Well, count me out then. That doesn't sound right. 10-15 mph winds are also enough to REALLY slow you down (I often wonder, if I'm doing 15 and there is a 16 mph wind, would I go backwards? I would in a plane.) I know a lot of the "serious" cyclists in St. Louis do the Tour de Cure in St. Louis, but I would be surprised to find myself at the back of the pack--that usually only happens on smaller rides.
Don't beat yourself up about it.
"Well-behaved women seldom make history." --Laurel Thatcher Ulrich
'09 Trek WSD 2.1 with a Brooks B-68 saddle
'11 Trek WSD Madone 5.2 with Brooks B-17