This. Also sit when you can. I’m guessing you are spending too much time in a crouched position on the bike doing almost a permanent half squat. That would be tiring. By riding more and getting better reading the trail, you’ll know when it’s OK to take short rests.
Regarding training, lots of core work, and leg work focused on single leg exercises. Pull ups and pushups also help (and other pull/push exercises)
Also, stating the obvious, it also depends on how technical the trail is. 1200 feet on a technical rocky trail and you will be tired.