Something that works quite well with younger runners is to have them breathe with a 2-2 rhythm for the first two-thirds of a race (e.g., first 2 miles of a 5K cross country race), then go to 2-1 or 1-2 for the final third of the race. If they can’t manage 2-2 for the first two-thirds of the race, it tells them they have gone out too fast, and next time they are to set an easier early pace.