Physical training and sports physiology is a field in which more information comes in weekly. It is hard to stay on top of the latest science. Of course, the single best exercise for riders is to ride, and ride, and ride. But there are other things that can be done depending on the difficulty of one's riding goals. My goal is simple. When I am in my seventies, I still want to ride 800-1000 miles each year. The two main things that can keep me from achieving that goal are becoming turned off from riding because of chronic pain and weakness or succumbing to a riding injury that makes riding much more difficult.
There is no way to completely control either unfortunate outcome, but I can increase my odds of still being in the saddle in my eighth decade by working hard to build a body that can do so.
Planks and other forms of isometric exercises help build endurance in the muscles of the core that make riding a pleasant experience. Without muscular endurance rides become painful for me after about the 50th mile. Conventional weightlifting with several compound movements adds strength to that endurance. These exercises make me strong enough to better remain in the saddle.
However, it takes more than strength and endurance to stay on a horse that spooks sideways at full speed or stops and reverses direction in the blink of an eye. Power, (which is not the same thing as strength) allows my body to respond to such movements with a lightning-fast counter move that usually keeps me in the saddle. Kettle bells, and backwards weighted sled drags have given my riding muscles strength, power, and a great deal of endurance.
But power from those fast twitch muscles, by itself, often is not enough to keep from being slammed into the trail. One also needs balance. there are scores of exercises designed to increase one's balance but too many of them are very difficult for an older rider to master. I found, (from studying the practices of an exercise innovator who operates under the logo of, "Knees Over Toes Guy") a simple, progressive practice that has greatly increased my balance in the saddle. I walk backwards a few miles each week in the dark. I walk a few hours before the sun comes up in my driveway. I started very slowly and very carefully. I have not yet fallen down. I now move much faster with deep, long backwards strides. I balance the movement with my arms.
These practices have made riding easier than it has been since I was a kid.
Of course, don't try any exercise routing without your doctor's approval. I am not a doctor. I am just an old man who has trained his body to ride longer and harder than I could when I was a high school athlete.