what's the terrain like overall? Is it a race like a muddy field or more hardpack tracks and stuff?
Totally depends on the weather. Like JV said, in 2007 we had sunny weather and hardpacked trails, which was great, but in 2006 we had the mother of all storms which turned the tracks to rivers and made riding the high spots on the course feel very exposed and uncomfortable indeed
The route tends to stick to nice bridleways and tracks though, with a few technical hills thrown in here and there to keep you awake.
There are road sections, but hopefully the course profile above shows that they're certainly not 25km of flat road.
As for choosing distance, again, you do that on the day... Weather dependant
This isn't a race, although there are people that will put the hammer down obviously. If you get good weather and you're riding with nice people, its a great day out.