Not so quick....... 2 months does not make an advanced intermediate. Group rides are about a lot more than how fast you can ride. There are certain skills you must learn and practice to ride safely and efficiently in a group. Most group rides will have a few groups to pick from and they will be categorized by speed and distance. For a rider new group riding I would always recommend starting off in one of the slower groups just so that you can learn these group skills. Sure the group may be slow for you but your there to learn initially. Once your pack skills are good then move up through the groups until you find your happy place...

To your other question about the 18+ MPH ride. If the ride AVERAGES 18 then there will be times that it probably hits 20 or 21mph and other times it may drop into the 15 or 16mph range.