Take a deep breath...

[HopAlongBunny]

...and then another, and another and....
That might actually be useful advice.
Breathing might be closely tied to arousal and control:

https://www.scientificamerican.com/article/meditations-calming-effects-pinpointed-in-brain/

Sometimes the simplest advice really is useful :)

[Seamus Fermanagh]

When I am on the treadmill or on a bike and riding, I generally pray Holy Rosary in time with each step/pedaling, establishing a consistent pattern.

On the treadmill, with it's monitors, I notice a 4-5% decrease in heart-rate when meditating when compared to not doing so. While unmeasured, my breathing seems more controlled as well.