Seven months is plenty of time to train up to that distance if that's your goal, but you'll need to purposefully train if you don't want to have a really bad time or hurt yourself. Your current fitness level and age will also be factors.

The main thing is that when you train for longer distances than your body is used to you need to slowly work up to the target distances so you don't injure yourself. The general practice is the 10% rule where you should only ever increase your total weekly mileage by 10% from the previous week. As a former shin splint haver and person who ran a half marathon last weekend, running injuries suck and take a really really long time to heal fully from. You don't want that when you're on a tight deadline, so best to be careful.

Generally when you're training for distance you don't want to do more than a single "long run" day per week so that you don't strain your joints and bones too heavily. There's a bunch of "couch to whatever distance" schedules out there that can give you a good schedule to start off with as a skeleton and, if you want, you can modify it for your body and timetable. Your long run is usually only ever going to be like 80% of the race distance max unless you're doing a race trial(which you should do once or twice in the last month or two of training).

I don't know how high altitude this is and I don't know if your body is used to the altitude that the race is at or not, but if it's not you'll also need to know that the lower air oxygen concentrations of a high altitude race will hit you like a truck if you're not ready for it. Just keep that in mind and it will make things much harder. If you don't have access to the altitude your race will be at you'll essentially need to train for a longer race than you're actually doing.

Hills suck. Hill workouts suck, but they also make your legs super strong, strengthen your lungs, and get you in shape very quick. Try to incorporate them into your workouts every other week or so in the beginning until you can add runs with decent hills organically built into them. Hills are a part of cross country running and you've just got to learn to power through them.

Get used to waking up early. As your distances increase and your runs take longer, first thing early in the morning is usually the only time to organically and consistently fit an hour or two of running into a day for most working adults if you've got a normal day job type schedule. Otherwise you'll just need to figure out what works for you.

Not a terrible decision if you can get lots of miles per week under your belt. Lots of running at low intensity to up weekly mileage should be more than enough with 7 months to train, given you've done 5Ks before. Even runs over short distances, if done often enough, will help. It's all about getting that weekly mileage up. What's your record 5K time?

7 months is lots of time, especially for running. And 15 miles is like, what, a little over a half marathon? Yeah the altitude might be a big adjustment but it's not hard to titrate yourself up to 24 km in 7 months.

I might suggest training a little closer to the full distance than you might normally otherwise to give you a bit of a chance with the altitude, but other than that you're fine if you train and stick with it.

(Also the stakes are pretty low, if the altitude gets the better of you you can just walk a few hills not that big a deal haha)

Good on you for pushing yourself comrade