UK Category 4 Road Racing
Try to figure out where the local racers do their training.  Ask your local bike store if they operate any training groups and if there are any racers in the group.  Go out with them on training runs to see if you can get a feel for where you stand in terms of fitness.  My LBS has rides once a week, and there are some racers that show up for that, but I don't see much of them after they leave.  I think this would be ideal, because even if you couldn't keep up, you could always hang back with the slower riders and have a good time.