So I am going to sign up for a 15 mile race 7 months from now. The entire race is at high altitude and a lots of elevation gain. I've only done 5ks before and I always have a lot of trouble with hills. I'm wondering what level of training I'm going to need to do to get better at hills, trails, and altitude. So far my road record for distance is 6mi and my trail record is 4mi.
Am I making a mistake or not (I'm doing this either way)?
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?
17:45
With a time like that, you could probably run 15 miles tomorrow in a decent time if you pace yourself appropriately. You have nothing to worry about if you get good weekly mileage in before the race. Just don't increase your mileage too quickly, that's a big cause for injuries.