Like marathon running, you need to acclimatize your body to the pounding your body will experience during a 24 hour mountain bike race. Along with pushing the pedals for a full day, the repetitive bumps are one of the biggest causes of fatigue during a 24 hour solo. Even with suspension you will still be getting thousands of jolts of various sizes during your race. With the right training you can train the body to tolerate the pounding with less fatigue. Think of the bumps as body shots in boxing. One isn't going to knock you out but thousands of them will certainly take the wind out of your sails. Just as a boxer has to toughen their body for those blows, the cyclist need to toughen up for the pounding we receive.
Here are three ways to improve your bodies fatigue tolerance to bumpy terrain.
You need to ride of course but cross training with Crossfit circuit workouts, trail running and punching a heavy bag will all help toughen up the joints and muscles. Not only will it help you with the fatigue from bumps but you will also perform better as cross training will strengthen the core and upper body muscles that don't get hit much on the bike.
Ride a Rigid Bike
If you race on a suspension bike then ride a fully rigid bike a few times a week on the trails. You will get more bumping which over time the body will build a toughness that will make it seem easy when you ride your suspension bike. During the lead up to your big event do an enduro or 8 hour mountain bike race on your rigid bike. I can't over state this one. I do most of my mountain biking on a fully rigid single speed and it has really improved over all body toughness and ability to resist fatigue both on and off road. A bonus is that your bike handling skills will improve as well as you need to ride cleaner on a rigid bike.
Bounding, jumping and tumbling. Plyopmetrics are explosive movements such as box jumps and clapping pushups. The goal it to increase maximal speed and toughen up the joints. Try doing two legged broad jumps for the length of a football field or doing pushups up a stair case (move hands up in a jumping movement and then walk you feet up the next step). The impact has to be built slowly but your body will adapt with stronger bones and joints.