Not sure I’d discount the descents. Yes, they are way less aerobically challenging than the ascents. But, you will usually have to descend in real life and so you need to get better and stronger at descending. I could care less about my descent times (used to love bombing downhill, not I’m slower to preserve joints), but I started including them in my training because it’s both important to improve at and also important to have an understanding of how quickly you travel. Even if it’s just to tell the S.O. when you’ll return. YMMV.
Also, I think 2 to 3 hours are the shortest “long” runs in some of the beginning plans. For many, 2 to 3 hours will be a lot for the body to handle even including the descent times. As with most aerobic activity, how quickly you recover will tell you how much you can add to suggested training times, but for now, I’d just trust these plans and Training Peaks, and stick to their times. It worked for me.