You’ll be issued with a map of the course area as well as a list of checkpoints which has a description of the checkpoint locations as well as a grid references. The aim is to visit as many checkpoints as possible in the 3hours. Each checkpoint is allocated points and your score is tallied up at the end. The map has some out-of-bounds areas but apart from that it’s up to you how you choose to get around your chosen course. The link gives you an idea of the area.
It really depends on you as to how you want to enjoy your 3 hours; compete for a podium finish or just enjoy a bimble around some trails testing out your navigation skills. If you only know the area from riding it then looking at it on a map may actually be a bit tricky as you have to relate what you see on the map to lay of the land.
Like Iainc says, it should be a hoot and then there’s the cafe for a bacon roll and some apres-bimble cake. Nice!