FWIW, when I could consistently maintain an 18 mph solo, I could more or less hang with a training ride that averaged 22-23 mph over a 24 mile route. It was tough at times, as keeping up wasn't just about pace. Knowing how to position myself within the pack and having the nerve to take corners faster than I would on my own also played a part. But given your crit experience, I'd say you'll do fine. Just be forewarned that some groups have no business riding as fast or as aggressively as they do. They act like they're a pro peloton but lack the skill needed. There have been some nasty accidents at least in Indy as a result. That, ultimately, is what more or less drove me away.