I would hack, hack, hack and then hack some more when you're done hacking. Getting out of the sandbox that is level and groomed will do wonders for both you and your horse. Get out on the open terrain, on the trails, in open fields - whatever, get out there on uneven terrain will help immensely.
Starting out at the walk, making sure it is a good working walk. Impulsion, forward momentum, swinging gait, tracking up, back lifting up into your seat, driving your horse into the bit.
Increase your rides from working walk, into a good working trot. Do lots of sets over time, increasing your time slowly each ride. Walk work, trot, back to walk, trot, back to walk - etc, etc - you get the idea :)
When you encounter hills, make sure your horse is working.
I do canter sets, they are valuable too! Make sure YOU work on your functional two point position, where you are staying out of your tack, but working just as much as your horse. Your lower leg, core, upper body - make sure you work on increasing your strength and longevity out of your tack, so you can be just as productive as your horse. Work on staying out of your tack longer and longer each time - so say if your BN course is 5 minutes, you need to work on staying out of your tack with ease, longer than that 5 minutes.
Welcome to the dark side :)