Never run a horse into the canter. You will have a strung out, upset horse. You need to develop a canter cue on the ground. I like to do it free longing in a roundpen because it is so easy that way. I use a kissing sound as an early canter cue. When the horse is in a nice, relaxed, cadenced trot "kiss" and encourage them forward with the whip. Pretty soon you can get walk/canter transitions because the horse will understand what you are asking.