Just to clarify, why do I suggest focusing on the gluteus medius?
Its bec its the area that supports yr hip and keeps it in position when yr other leg is in support position (ie when yr bad leg is in forward swing phase) while running.
When we consistently put pressure on that particular leg, as when we run on track curves, the hip and pelvic bones converge and compresses on yr left side (when making left turns). Over time the nerves on that side go dead, resulting in the loss of neuro-control over the gluteus medius (and some of the other related muscles there). This can lead to sciatica, numbness, stiffness initially, esp when u r seated.
Over time, the gluteus medius atrophies and this results in the pelvic tilt downwards to the left. Surrounding muscles compensate by shifting or working harder, and this makes running possible for a season, until one day u start to feel yr right leg sway and splay outwrds, bec the compensation can only go on so far.
U will notice 'smaller' or weaker muscles on yr right, and try to strengthen them. But they seem not to respond that well. The more u run, the more yr left leg seems to grow larger and tighter around the hips. However, when u stretch, esp one leg at a time, the left is easy but it is yr right that tightens! PERPLEXITY.
But if u get strght to the ROOT PROBLEM, and deal with it, the other muscles automatically adjust to cope with the physical changes in yr left hip abductors. Strenghthen the glute med of the leg with the lower pelvic tilt. Then the other muscles will adjust in time, as u continue whatever form of running u can muster.
When I first started with strengthening my left glute med, I noticed a change in the level of flexibility on each leg. When I swam I also kicked more fluently, and held a better posture (oh yeah swimming is a great way to assess yr level of alignment and progress in balancing out yr body, not to mentioned good x-trng if u really can't run at all!). When u r in the water, the cooler environment enables u to have a better 'feel' of where yr trouble spots are, and the limits of yr range of motion.
Don't worry so much abt the other leg (or any other parts of yr body)and its resulting troubles due to compensation. Work on them, but my suggestion is focus on the left glute med/hip abductors and everything will fall into place.
It also helps if u wear minimalist shoes/racing flats for yr current runs, or do barefoot runs on grass. They won't necessarily heal yr condition, but allow u to continue running with lesser discmfort compared to cushioned thick trainers. U also get a much better idea of where yr problem areas lie and how much progress u r making in yr rehab. Softer surfaces also give more easily and makes yr imbalances slightly less interruptive to yr runs. But this is just provisional and by no means a cure.
Strengthening yr weaker glute med, not overdoing track curves (make right turns too!), not using track too often.....these will help.