Too few people coming into riding take into consideration how important muscle tone is in horse riding. Many people get the idea that because the rider is sitting down and the horse is doing all the work, Newtons force of motion and gravity don't apply but they do, and the rider's body has to be fit and tuned to counteract the forces involved.
If I were to be starting out again in riding, I would visit early on a sports physiotherapist for a complete survey of my skeletal structure and muscle tone. I would expect that the physio would outline an individual exercise programme for me. I already know the Pilates exercises work on the centre core, the balance and the stretching of muscles and tendons. As a minimum two sessions of Pilates a week must help the aspiring rider.
Personally I was lucky the sports I had done previously (scuba, swimming & rowing) work on many of the muscle groups used in riding, although my Pilates instructor occasionally nags at me for using the wrong muscles and for breathing in when I should be breathing out.
Relaxation is also important and reading up on Dr Alexander's techniques can help the rider to learn how to relax.
Of course it all depends on how far one wants to go in this hobby of riding. However to sit a horse in the correct posture for a couple of hours for a hack calls for a degree of physical fitness, make no mistake.
It comes as a disappointment for me personally to look at professional male riders who always seem to be short, thin, wiry, chaps and I am none of those things. Which is probably one reason why noone would ever call me a stylish rider.