A Few Considerations

I like to think about learning in a spiral. We learn basics at the bottom of the spiral and as we grow in the learning and move up 'a rung', we learn a little more about each of those same basics so that the basics gain in dimension.
Our first lessons on a horse may be about sitting with the ear, shoulder, hip and heel in alignment, how to steer and ask with the legs. This is normal. Over time, we get more proficient at holding this alignment and in the steadiness of hands and legs. Still over more time, we learn more of the 'whens' to use hand or leg and aiding develops greater specificity and complexity.
Something else happens as we spiral upward in learning. Our focus becomes less and less on what our body parts are doing and is directed more at what the horse is doing. In some interesting way, I liken this to a sort of 'maturity' on the horse. Without some view into how this 'maturity' affects our riding, we might never realize that the highest levels of understanding and communication with the horse are going to require much greater self-control, especially control of the mind.
It is more than 'mental focus' that we're after though. It is a higher level of selflessness, a diminution of ego, that puts us into a state of being where we can partner with the horse the best. We will need to give ourselves over to the horse, over to the partnership. At this point the mind is so emptied of the clutter of concern about the self that it can direct the body with speed, accuracy and clarity as the body relates to the horse in the partnership.
Adjustments in posture and influence need to come with such speed that they are almost imperceptible. Practice of the Tai Chi form is very helpful in this area. Qi Kong exercises, meditation and even prayer can develop this special mind control.
Breath control and lung fitness are important as well. Why is this? Apart from the fact that riding can provide a mild form of aerobic exercise, requiring regular full breathing and stamina, the quality of riding I have in mind demands a working understanding of 'energy flow'. 'Energy flow' is related to breathing in many special ways. Tai Chi is good for teaching breath control as it relates to, enables and influences motion and energy flow.
Body control for the dance between horse and rider is less about strength and more about muscle tone. Developing strong muscles may be a disadvantage in terms of energy flow and finesse. Muscle bulk is excellent for lifting but can 'trap' energy that needs to move around....or stir....within the body.
For those who have poor muscle tone, the ability to create tone as required is sufficient. Either way, be it a well toned body or poorly toned one, both will need to be able to alter the tone in specific areas to create minute signals to the horse and to move energy(or chi).