I did my first cyclo cross race of the season last Saturday. I really enjoy the cross races and the whole experience that comes with it. The racers are more laid back even though the races are much more intense on the riders than road races or crit races. It is also a safe way to add racing to your riding. Some race venues are much like a tailgate party in many ways. People cook out, have a few brews, make little insults as you ride (when I helplessly laid over in the sand pit on Saturday, I can still hear some spectator shout "Bill is down, Bill is down"), ring cow bells and even wear Halloween costumes.
I have a Trek XO1 cross bike and three different sets of tires to match the terrain and soils we race on. Three years ago I raced my son's full suspension MTB in the 3 races I competed in. The MTB will work OK but will be a hindrance when you need to carry it over barriers, stairs or across a ditch. At last weekend's race the first two place finishers were on 29'ers, in both my race and the expert races, they just rode over the big tree roots and loose dry soils that slowed down the cross bike riders.
Cross racing is a "sprint finish followed by a time trail". A really good warm up is critical. At the beginning levels there is no pack riding or team tactics. My recommendation is for you to race this week. Waiting a week or two for the next race will allow you to do what? Racing this weekend will introduce you to the 30 minutes of pain and suffering and allow you to find what skills you will need to develop to race better next time. Get to the venue early and be one of the first riders on the course. Most races have the beginner races scheduled first which enables the riders more time on the course. The race I just did had only 15 minutes between races for us to ride on the course, which amounted to one full lap and part of the 2nd.
While racing you will question yourself as to why you are doing this. It hurts, you are out of breath, you probably have crashed two or three times and you still have 10 minutes of the race left. About 15 minutes after the race you will be looking forward to doing it next week. That’s OK, it’s normal for a cyclo cross racer, just ask the guy next you on the start line

. Also, there may be cyclo cross practices in your area during the weekdays, ask around.