I would also make sure to incorporate some running on sand if possible. Also, you can add stairs, hills, grass and off-road running as well, because it’s a great way to challenge yourself and improve your condition even more. If you are brave enough or in a good healthy state you can add a backpack while you are running so you can increase the strength on your legs and of course your stamina and aerobic capacity. Since you are already a spinning instructor and you have completed 3 half-marathons so far, your body should be conditioned enough to follow some of the ideas I’ve mentioned. Do it with progression and always listen to your body so you can avoid injuries.