I have my biggest cross country race of the yr coming up but think I have cuboid syndrome, what can I do?

Depends. What you do in this situation is dependent on how much time you have before the big race. In brief, it needs rest, modalities to reduce inflammation and alter loading or conditioning. If the race is in a few days then aggressive medical management with anti-inflammatory medications may save your race and the preparation you have made. If you have more time, then greater options are available.
Try manipulation. You may also want to see if there is anyone in your area who is skilled in manipulation for this problem--if they can get it back in place your pain should go away quickly. You can look for physical therapists or sports medicine physicians for example.