Don't be anxious about it, in my group last week about half of them were absolute total novices off-road. I got the impression that a number of people on the course were quite new to motorcycling in general; one guy managed to drop the R1200 en route out of the car park at ORS. If you're comfortable on the road on your GS then this is a big advantage. Being used to standing up on the pegs helps because you'll be doing this all day.
They go on *a lot* about how you'll be wet through with sweat by the end of the day, but I found this to be total bollocks. I was freezing most of the time, and I didn't find the exercises that strenuous. I kept the liner in my jacket, and two thermal base layers on. However it is undoubtedly a muddy place, so take gear that can get filthy. A lot of the participants turned up in what looked like brand new full MX gear; a bit intimidating until you realise that they've just gone out and bought all the stuff the day before, and have never used it in the wild. I'd suggest organising your accommodation as soon as you've booked the course, I didn't and ended up staying in Swansea which wasn't a great idea. They organise a group meal on the evening of day 1 but enforced group jollity with strangers isn't my idea of fun and I didn't go - but if you want to take part, you probably need to stay near or at the venue (the Abercrave Inn).
One more thing - if I was doing this again (I'm not) I'd take either a tailpack or a small backpack, useful to keep a small bottle of water in, and to add/remove layers if you do get cold/warm. You do not need a camelback. They say their van travels up to the site, but most of the time you're not near it so it's better to be self-sufficient.