I'd agree with all of the above. Especially the pack riding/group skills thing. That's way more important than your average speed, IMO. Don't worry about how you finish. From what I've seen the other racers will rag you about being a hazard on the course, but not about how fast you were (or weren't). If you get peeled right away, you can bag the race, go home, and plan your winter training.
I rode my first race this year. Here's my advice, which is based pretty much what I was told.
Go for a road race to start with, even though crits are more common. Expect to be bumped and shouted at. Don't take it personal, adrenaline and testosterone are running high, and there's a lot of good equipment and flesh at stake. If it's good advice, take heed. Plan on going to learn, not win, at least for now. Coming down to the finish, if you're in the pack, just sit up and watch. Charging to improve your finish a couple of (meaningless) places is a common cause of cat 5 wrecks.
If you have the group experience, you're in reasonable shape, and there's a race or two nearby, go do it. It'll help you know what to expect and how to train for next year.