Sounds like you’ve got some fun objectives coming up! Some good questions on training: because you’re venturing into more of the mountain running realm it’s useful to think about not only the mileage of your target event, but also your expected duration to finish, AND the vertical gain required. In this way, when you’re planning your training you can account for those measures and make sure you’re being specific with training.
I would definitely put increasing emphasis on building up vertical gain and using an incline trainer is a good, albeit somewhat contrived way of doing this. You can supplement some longer aerobic sessions at a steep grade (20+%) with one session per week of short, powerful hill sprints (start with 8x 10sec, 2-3 min rest) to build leg strength. Then you can extend the duration of the “vert” day, and eventually look to building in some moderate intensity (tempo/Z3-level effort) on incline to better simulate the race demands. Maintaining some flat/rolling terrain in your diet is important, though, as your 15km race sounds like it doesn’t have the degree of vert and therefore the race speed will likely be higher.
Regarding the macrocycle length, i wouldn’t worry so much about the total number of weeks. Instead, I would try to work into a 3-4 week “build” followed by a good rest week, dropping volume/mileage at least 30-40% and focusing on recovery modalities. So long as you’re managing your energy and making sure your body is recovering well between quality sessions, you can carry this rhythm up to within about two weeks of your main objective, then begin tapering off volume to sharpen for the event.
Hope that helps – good luck!