If you are up to the stage of competing it is vital that your fitness is up to scratch.
There are so many times that you see riders out and competing and when they finish they a blowing and puffing more than the horse is.
If this is the case there is no possible way that you will be able to get the most out of you or your horse.
We often get so focused on our horses fitness and training that we forget to make sure that ours is up to the same level.
If you find that is something that is happening to you then it is best to get a strategy to increase your fitness.
A combination of cardio and weight training is best to get you up to the adequate fitness level. Going to the gym, getting a personal trainer, or just doing at home work outs will all get you the same result. So just do what feels comfortable for you :)