I too am interested in hearing more about how others have developed standing MTB fitness. I’ll climb both in and out of the saddle for 2 hours and feel like it was only moderately difficult and then in 1200ft of technical descending my legs are burning so hard that I can’t fully enjoy it.
I’m sure most of the fix is training, but I’ve also noticed that others vary their standing position on descents to conserve energy - going between more of a straight legged stand on smooth sections and then full attack position when needed.
This being my first winter using a trainer, I feel like I will get really really good at sitting and spinning, but without the standing it may not translate well to the outdoors. Definitely going to need to start lifting heavier or doing circuits as mentioned.