If you want to increase your endurance then ride until you are very exhausted today, then tomorrow, ride that same distance and a little more.
If you want to get better at technical terrain then you’ve got to practice. There is no other way to get better. In order to make your practice the most beneficial then it would be best if you had a friend with you who could watch what you are doing, and so you could watch what they are doing. It also helps if your friend is better than you, so if you are going on a trail together you will be forced keep his pace.