Fitness
On a Camp 404 expedition you'll spend three days carrying a loaded backpack up and down hills along rough paths. Preparing for that will involve training. Lake District expeditions are the most physically demanding, but you don't have to be super fit to join a Dartmoor expedition. If you are able to walk a few miles now, you can do it. You just need to spend time building up your fitness.
The time and effort you invest in getting your fitness level up and pack weight down will have the biggest impact on your expedition experience. The earlier you start the better!
"The pain you feel today will be the strength you feel tomorrow." – Arnold Schwarzenegger
Start by making a training schedule. Doing this about 6-8 weeks beforehand will give you enough time to make a real improvement to your fitness. Set some goals, but make sure your schedule is realistic.
Build up your training slowly over several weeks. This can be a combination of walks and exercises. If life is very busy or it's just hard to get out walking you might need to rely more on exercises. That's fine, but doing some long walks is essential to allow your body to adjust and to make sure your boots and other gear are trail tested before you go anywhere near the hills.
Doing regular training walks is a good routine for a healthy mind and body anyway, and it might become a habit you don't want to break.
Getting fitter will make you feel more confident about going on Expedition, and the fitter you are the more you will enjoy it.