how different is riding a mtn bike?

[RoadRaven]

Speaking as someone who prefers road racing, I do have a mountain bike, and sometimes ride in the paddocks around the farm. Nothing technical... mostly flat... so I can't really help with "riding a mountain bike with others or in an event" advice.

I also have NEVER ridden (or even driven or walked) in a desert...

But... you asked about the difference between riding the two, so here's my two cents worth.

Riding the mountain bike takes longer to cover a distance, its harder work...
you have wide tyres, grass/dirt/moisture,
you have knobbly bits on the tyes,
you encounter soft patches (like sand) and suck-you-in patches (like mud)
you encounter leaves/sticks/stones...

The difference in tyes and riding surface means greater resistance which equates to harder work - even if the course was flat and straight like a road, on a grass surface with wide tyres, you will not match the speed of a road bike on a road.

The terrain, even when flat, makes it more technical because you are avoiding obstacles like bumps, stones, sticks... This all leads to more concentraion and, like Kali says, more fatigue...
Its loads of fun, but I like speed which is why I prefer the road.

Alot of elite pro-cyclists did their initiation off-road... I would suggest you try and borrow a mountain bike - they do feel different - plus the gearing can be different (espesh where you change gears on the handle bars). Miles on your road bike will of course be good, but it would be useful for you to "get a feel" for a mtn bike before you are on one in an event. Like Susan says, its quite a different work-out and you should be prepared for what to expect.

All the best - I'm sure it will be loads of fun.