i agree with the above, and will repeat what i tend to say for all similar situations like this. trot, trot, and more trot. and then when you think you are done, trot some more. better canter comes from better trot, not from working more at the canter. trot builds muscles and balance while cantering burns calories, so by focusing more on the trot, you will actually be setting your horse up for a better canter.
limit canter to just a few strides max per ride, and focus on trot sets. a great exercise is to change diagonal every few strides (on the straight) and see if the horse stays balanced and even. if they lose rein contact, pop their head up, or something like that when you change diagonal, it indicates a weakness. when your horse can tolerate your change in diagonal every 3-4 strides with no change in their way of going, that's a pretty good indication that you have a balanced horse that is moving through their topline. be sure to have a soft, correct contact on the bit because if the horse is leaning, than this exercise is useless. but done correctly, this is a great indication of fitness and conditioning and when your horse can move up to regular canter work, as well as starting OF.