Symmetry Under Saddle is a rider core fitness program that is effective and scientifically proven through horse and rider biomechanical research. You can find it (and more information on the research if that part interests you) at https://happyathlete.ca/downloads/ It is great for riders of all disciplines and abilities...:)
I am a horse and rider fitness coach. So, I'd recommend my programs :D
www.leslieklatt.com I have 30 and 90 day programs, plus 5 day bootcamps.
You can do strictly rider fitness, or a program that works on you, your horse, confidence, goal setting etc etc.
30 day challenges plus add on the water challenges and healthy foods and recipes #pinterest
I've done the 30 day challenge from strides for success! It really did get me in better shape! It wouldn't help me progress very much anymore (I did it about a year ago and have since moved to different workouts), but it helped me get into basic shape and get a routine for fitness! It may not be the most strenuous thing if you already workout, but it's a great place to start!
I've heard of Lorna strides for success.. don't know much about them though.