You won't be able to gauge your ability to compete in a road race by average speed. Road races consist of generally cruising along in the pack, and then responding to attacks and accelerations. The pack may go from 17 mph for several miles to 23 mph for several miles. Go spend some time riding with your club. You'll learn a lot in a short amount of time on club rides. In fact, I wouldn't recommend racing until you've done a few club rides.
In your first race, don't worry about anything but finishing. Just watch how things go, see what it feels like to be at the front, and be in the middle of a pack. Whatever you do, don't get dropped! Stay with the pack. You'll soon find that being able to survive the near chest and leg exploding intensity of an attack is crucial to being a good road racer. Concentrate on being smooth, don't make any sudden movements, and don't slam on your brakes. Pay close attention to everyone around you, since they are likely racing for the first time as well. After you get some racing experience, you can try different things during the race and try to win.
Road racing is about putting in max efforts at the right times and conserving as much energy as possible.
To conserve energy, stay out of the wind! It doesn't matter if you're cruising along in the pack half asleep because the pace feels slow, or if you're feeling extra strong. The only reasons to stick your nose into the wind are because you are riding for someone else, or you are going for the win. You might feel like you're cheating, or being a lazy slacker, but you're not! (If you're strong enough, you may be able to break away from the pack and win solo or with a small group, but that will come further on in your racing career.)
Whatever you do, don't expend energy unless you absolutely must. There's no reason to be the first over a hill, or to be in front in a headwind section, unless you have a specific goal and purpose to do so. Save it all for the end. The person that wins the race is someone that: is fit; has conserved enough energy for a full sprint; is smart and has good placing in the group; and has a decent sprint. The best pure sprinter will not always win.
Remember to eat and drink every 20-30 minutes during the race, even if you don't feel hungry or thirsty.
Cycling is a very subtle sport, and to make matters worse, has its own language. You'll be on the rivets when one of your teammates tightens the screws if you're knackered, but will be able to put the hammer down when you're going well. Communication among cyclists, unless they're angry and yelling, usually consists of eye movements, head nods, and elbow wiggles. Watch a couple of the pro 1/2 races if you get a chance and you'll see what I mean.
Most importantly, have fun.