As other’s have said skills play a big a roll as does overall fitness. To get more specific these are things I wish I would’ve spent more time considering—it’s taken 2 seasons (or about 15 races to figure this out).
Turning—this is probably one of the biggest differentiators between poor and good cyclocross racers. Start your braking before the turn, not in the turn and try to hold as much speed as you can going into and through a turn. This is a comfort thing and you won’t be able to perfect it in the first race. Carrying as much speed through the turns allows you to not have to hammer out of the corner.
Tire pressure—run as low of pressure as you can w/o the risk of flatting. For example, I weight 180lb and run my Schwalbe X-Ones at around 21-22 psi. Lower pressure = more grip. Grip is good.
If you can get out to a local park and do some practice.
Don’t blow up too early—because you’re Cat5 you’ll be racing for 30 min, pace the first lap or two, and then adjust based on how you’re feeling. Staying near the front allows you to avoid the yo-yo effect that mid-pack and back of the pack riders face. Given it’s your first season racing you’ll be placed at the back or near the back of the start grid, that’s ok, move up if and when you can.
You always have just a little more in the tank—there’s nothing worse than ending a race thinking, “Yeah, I coulda went a little harder.” Leave it on the course.
You’ll learn more as you race more. Drill the skills like others have said, but also use your first season to learn. Try different lines, run sections, ride sections, try it all. Learn as much as you can about how a bike handles, what it feels like to push the grip-envelope of your tires, and how it feels to disconnect your body from the bike.
Regarding fitness, I think some indoor workouts are good prep, but I think race-pace hot laps at a local park are better. Nothing on the trainer can replicate the demands of a CX race.