Oh, I don't know that my advice has been any good. Hubby (who regularly averages 15-16 mph on a hilly-ish ride) just told me that he can find people in the green group that go "plenty fast". But...
I should mention that he is kinda an, um, oldish fart (I am guessing in comparison to you) and when he was a youngish fart he probably would have been riding with the CATS group, no doubt. He also said that the only reason he wasn't riding with the CATS group now was that it was too much of a logistical challenge to get both of us to different start locations. So, he's holding himself back for my sake....
You are young and strong, so maybe you should go for it? (and be sure to print out the map, and, and, and...)