go back and go back. 18-20mph means an avg.
i got dropped from every single group ride 3 years ago. every single one. then i learned about the mental part of riding and how to ride smart.
now i am out front, pushing the pace. some rides i feel good, others you just hang in the middle of the pack.

the only way to ride @ 25mph is to train @ 25mph. if you start with them, last as long as possible (learn as much as possible) then continue with your miles, you'll do fine.

always take the opportunity to ride with better riders. be patient. begin hydrating 3 days prior (pee every 3 hours) and load up with pasta sandwiches, and really learn about yourself.

this ride seems like one big interval. you can go 18-20mph all morning, but haven't been trained to ratchet up the mph. if they are avg'ing 18-20mph and there are points where they go 27mph, then math will tell you that they slow up and cruise for a while in that ride. once you learn use intervals in your training, you will hang with them.

if this is the type of ride you want, then train at those speeds.