Racing is the most fun you can have on your bike, aside from chaingangs. My highlight of the year was standing on the start line next to Lizzie Armistead and the Wiggle Honda team
As others have said, learn how to chaingang and get used to riding close to others, avoid bad wheels, stay off the brakes, don't half wheel, don't touch your garmin or get distracted, hold your line in corners and don't move suddenly without communicating, gels are your best friend in a race, use your drops if you can - less chance of bars getting locked if contact happens, and stay near the front if possible.
Your first one will be a shock to the system i'd imagine, especially if you pick the first race of the season to start - they are generally a little more nervous and twitchy than later in the season. The pace will vary depending on course and weather. Depending on the lead car driver(s) you may have a nicely paced neutralised start, or an eye balls out start, and even more eye balls out effort once the flag goes down. I've lost count of the crashes i've seen in the neutralised start. Oh, and warm up well too, turbo or rollers are better than riding on the road.
Crits are totally different to road racing, sprint, corner, sprint, corner and repeat for 40+mins!
If you are a vet, you can race the vet categories which (ime as they put women in with vets for some races, and i think the men's vet categories start with 40+ and then 50-70+) is far safer than 4th cat RRs and a totally different race experience. I've raced with the 40+ and 50-70+ there's a little difference in speed (but that might just be weather/course profile) and there's some blooming fit old guys!