A good way to start varying the stride within the pace, is to use the counting exercise.

If riding forward in a balanced active Working trot, count the steps you take between the quarter markers, eg F-M or K-H. Do this three times and make sure it is the same every time. This will encourage regularity. Then ride the same lines with one less step, so you need to ride forward longer stretching the stride. Then ride the line with 2 extra steps which will shorten the horse a start towards collection.

This can be done in trot and canter and you can play around with the number of steps you demand depending on the ability of your horse and, the skill of the rider.