I'm considering going spinning in the local gym in preparation for dusk til' dawn and the torq race solo, I already ride 18miles a day to work on my fixie and 3 or 5, 2-4hrs xc rides a week so i'm wondering if it will have an effect on general riding fitness?

depends if the tussi in front of you looks good in lycra…

I already ride 18miles a day to work on my fixie and 3 or 5, 2-4hrs xc rides a week

You're going to win. Relax and eat cakes, cheese and drink beer heavily until the event.

For what it's worth from my experience, spinning is great for sprint training, as the class would only be 50 mins of balls pedalling. (and the view of the ladies was very nice too.)
If you're already doing that much on the bike with the work commute and the xc rides, I wouldn't have throught it would improve your stamina in the slightest. Which if you're aiming for solo training is what you'll need more of.

Still a good work out though.

alpin, +1.

Good workout, if your fit you could probably do 2 sessions back to back 😀