I have a 2006 Raleigh Mojave 8.0, which I purchased in 2006. I have ridden the snot out of it, i've taken on some good trails. I've considered upgrading to a new bike, mostly so I can ride with other people that have fancy bikes. It's not in my budget right now though. I was recently surprised that I was able to climb better than my friend on his full suspension XC bike, but he smoked me on the downhill (I understand the shock absorbing the pedal strokes, but I thought his higher end components and lighter bike would make him significantly faster than me)
I've been running half marathons and marathons a few a year for the last 7 years. I understand foot races. I've been interested in shifting to something new, so i've started investigating MTB racing. I won't win any races, i'm just looking for a good time.
1- I'm afraid i'll be behind the curve because I don't have a fancy bike, I'm a fairly strong rider, but a bike can make a difference.
2- I don't know the racing culture, foot races are very open to people just racing to complete as an achievment.
Really the biggest thing is #1 , i'm afraid my bike will set me back and If i'm going to race I don't want excuses, I want to be competitive for my skills and strength and not my wallet.
Don't over think it. Once the race starts it will come down to the rider, lungs and legs regardless of what your sitting on. The bike, assuming its tuned up and functioning correctly, won't hold you back much at all. If you want to do something to help you feel faster on your current bike look at some good lightweight tires and or wheels if you can swing that as they will have the biggest impact. Most local races will have different classes you can sign up for Expert, Beginner, Sport etc.. so you can potentially race with others of same ability.
I just started racing short track this summer on my '98 Gary Fisher, and it's definitely not the bike that's holding me back. Nobody cares what I'm riding and the scene is very friendly and inviting. I say go for it and have some fun.
There are a lot of different kinds of MTB races. You can race 24/7, xc, enduro, downhill, endurance, pretty much anything under the sun. If you stick with xc or endurance races, a HT should work just fine. I find that MTB racing is a lot more friendly than road racing. It also tends to be less dangerous. Have fun!