If this happens again, you might look into getting a battery with a higher reserve capacity. I don’t know what your Subaru calls for, but the stock battery might be a little undersized for that climate. Also, maybe try another brand of battery. While it’s true that there are only a few manufacturers in the US, there is a store near me that’s notorious for selling lousy batteries. Not sure who makes the batteries for them, what they do that’s different, but everyone I know that’s had a battery from them has had to replace it prematurely.
In the cold AK climate, especially since you are probably driving with your headlights on for a good chunk of the year, and no doubt the heater/defogger/fan/wipers a lot, you do indeed need to give the car a good run a couple times a week to make sure the battery stays charged. If you have a source of electricity handy near the car, it might be worth getting a “battery tender” or a small charger. You could probably keep it hooked up and just plug in the car when you don’t get a chance to drive it much or know it’s going to be sitting overnight on a particularly cold day. A block heater of the type that goes in the oil dipstick tube might not be a bad idea either. I’m assuming you already use full-synthetic oil in the car, based on the climate. If not, you should be.