A big part of what we do (Outside of the gym) for MX conditioning is MTB, I advise you get familiar with Enduro to build endurance, Downhill (Speed and corner control, Jump tracks to keep your Whip game strong.
The correlation between the two sports goes way beyond the fact that they both have two wheels.
On an MTB you will have to condition your heart, lungs, and legs all the time negotiate a challenging terrain.
If you can, find some trails for the mountain bike. This will definitely help you with your dirt bike riding.
The more you practice this, the better you will be. Keep checking back for some great conditioning tips, Nutritional advice that will help you keep those wheels turning.
Product I advise you adding is Holeshot - Due to its high electrolyte complex.