You signed up for it because you know, somewhere deep down, you can do it I've never done an obstacle course, however I was nervous to sign up for any race at all because they are intimidating. Once you do it, you'll wonder why you were so worried in the first place. Get your gear ready, get your music turned up loud and go for it - and make sure to smile because you'll have some great pictures to show off when you're done!

You'll be fine if you Crossfit.

Do try trail running once or twice a week leading up to the event. Go slow, and most importantly, don't injure yourself. (Running efficiently is sometimes about taking lots of small steps, rather than big ones). I run with a friend who knows more about it than me, and that really helps. The running will really help mentally, because you need to know you have endurance, and some mental techniques for pushing on when you want to walk. I just try to go a bit blank, and stop thinking about how far is left to go. I just think about taking the next step. (Although, in the event itself, by all means walk for a couple of hundred yards if you have to.)

I found Energy gels to be helpful on a Tough Mudder I did recently, gives you something to look forward to every hour.

There's a post somewhere here about clothing choices, obvious stuff really, good woolen socks because your feet will be wet, lycra or similar top and shorts for the same reason.

Don't train for a few days before the event, eat really well the night before, lots of carbs.

Most of the obstacles in these things tend to be about self belief. Out of the 30 or so we had, I genuinely thought I 'd fail on 5 or so (and look like a fool to boot). Take a deep breath and just try, you'll surprise yourself how much more you have in you when it matters.

Good luck.
You can do it because you're already training.
That's the key to success.