I have mental conditions, it's really not... safe... to be away from horses too long for me. The one time I took months off, turned into one of the most monumental events in my life. Not all that fun. Even now that I have my horse on my property and can see her every day, at least for basic care, I start getting real anxious and grumpy if I don't ride. It isn't simple exercise, because I sprint, jog, and do jumpstyle dancing to help drain out my energy and cope. It's everything about riding, the feel, the communication, y'know... Once you get a taste for it, you're hooked!
Nice thing is, I remember being a freshman in highschool and having to take leave in the middle of class to sprint up and down the courtyard almost every day because my panic attacks were so severe. I would start hyperventilating and my heart rate becomes psychotic. So the anaerobic exertion would force my body to re-adjust. But, in the past four years, I have sprinted about 3 times to literally run from my PA's. It's a quick fix. Horseback riding 6 days a week is what makes the difference, I consider it part of my medication regime.