Agreed with the above folks.
Sounds to me you should start with the B group ride. Every week. If you find it slow, focus on group riding skills and being super smooth. When not riding in the group, get used to doing intervals that make you want to cry, chew your bar tape, cuss, and maybe vomit.
After a good while, move up to A group. Keep at the training.
After a good while, move into the "race sim" group ride once in a while. Get dropped, try again. Get dropped, try again. Finally hang on. Eventually toss in a few pulls, then someday maybe even an attack.
Race....
None of that but the training and solo riding is necessary for TT if you want to try that. Cross can be as bumpy and jarring as MTB is depending on the course.
IMHO, the basic entry level of fitness in beginner racing has jumped a bit in the recent years of Zwift and "structured training for the unwashed masses" kind of programs.
But, it can be fun and rewarding. Just keep your wits about you and have small incremental goals (one of my biggest problems).