marathon is all about the distance, therefore you have to have very good endurance base.
The best way to train for anything from a marathon to working as a rock wall construction worker is to assess the current fitness level, assess weaknesses needing to be addressed (which includes muscle imbalances, postural issues, musculoskeletal issues, etc.), assess the activity biomechanics, design an initial program to use as a starting point, and move on from there with progression/regression, program adjustments, etc. Each client will have different needs. While there will be some similarities and exercise design crossover, we need to be focused on treating each client as a new and separate individual.
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.