I went to a fitness class for riders and while it was 'kind of' useful, most of it was devoted to core conditioning. While I could always use more core, I am finding I need to strengthen the muscles that 'shut off' when I ride english.
Specifically I am thinking that as an (older) rider, I do not engage the internal rotation of my hips. To demonstrate, sit tall in a chair, knees together, put a pencil between your knees. Using an elastic exercise band, wrapped around your ankles, and now try to move your feet away from each other keeping the pencil in place. The angle should be at least 45 degrees away from each other. You will feel this in the deep hip rotators or the tfl (tensor fascia latae). I think this contributes to my uncomfortable hip pain after I ride, as these muscles do not fire because they are the antagonist to the ones that do fire, (the adductor group).
I am always looking to find that magic stretch that will take care of all my discomfort and I'm still looking! But I think my theory is worth working on.