12 Hour Adventure Race
Many adventure racers, after doing a few sprint races, long for an even greater challenge but (wisely) don't want to jump straight to a 24 hour race. 12 hours is a common intermediate length for an adventure race. There are some important differences between a 12 hour race and a sprint race. Though the disciplines will be pretty much the same, each leg of the race will be much longer and you will have to carry more gear, food, and water which will add to the difficulty. Common distances for a 12 hour race are 6-12 miles paddling, 8-14 miles trekking, and 25-50 miles biking.
In general the difference between a sprint race and a 12 hour race is that race directors feel they can beat up on the racers a little more in a 12 hour race because it is assumed that most racers have some experience with endurance competitions before attempting a race of this length. So if you've done a sprint race and felt like the race director was handling you with kid gloves, then you might want to give this length of race a try!
Nutrition, which plays a more limited role in sprint races, will become much more important as it is easy to end up bonking if you don't take in enough food and water. The disciplines will generally be more technical in a 12 hour race, with the orienteering being trickier as the race director will assume entrants have a fair amount of navigation experience. You may be required to plot your own checkpoints on your map and you will probably be going off-trail much more than in a sprint race.
Also the mountain biking tends to be on more technical trails and the race director may utilize more singletrack if there is any available near the race site, which is less common than in sprint races where most of the trail biking will be on wider trails or fire roads. Part of the race may be in the dark which introduces extra gear to carry (lights, extra clothes) and extra challenges as checkpoints will be harder to see and navigation is much more difficult.
Mastered the 12 hour race? Then it's time to go for some serious bragging rights by attempting to finish a 24 hour race.