I know this isn't strictly an MTB event, but if you've got a road bike (or a road-friendly MTB), and want to try a challenging event with a friendly atmosphere, some friends of mine are putting on their first Audax next weekend.

For those who don't know, an Audax is a self-navigated timed ride, usually held over a long distance (this one is 60 miles easy route and 120 miles hard). You can either beast yourself trying to get the fastest time you can, or just enjoy a hand-picked route on quiet roads with lovely views, and eat as much cake as you can cram in.

Entry is cheap (six quid), and from the bits I've ridden of the route it should be a really good event.

More info here:

http://lvis.org.uk/files/audax10info.pdf