I would recommend you to schedule your training on a week or a month to alternate the exercises you'll do with your horse and to adapt it to his state of health, to the weather and other conditions...
It could be a good idea to plan sessions of trotting and cantering two or three times a week to improve the endurance of your horse. Try to focus more precisely on the competition, by keeping about 2 sessions of dressage/jumping/cross-country according to the discipline of your event.
Share your time to avoid the repetition of the exercises and keep your horse stimulated. You can train your horse on the lunge for 20-30min and work on the flat to finish the week.
Don't forget to reward the good work and to pay close attention to the needs of your horse because after all that how you'll know how to organise your training sessions ! :)