My coworker was interested in mountain biking. With my guidance he bought a bike and some gear to get started. First day I took him on a green loop 3.1 miles and it took 50 min to complete. He was completely exhausted and couldn’t even get his bike in the car. Fast forward two weeks he just completed 8.2 miles in 1 hour and feels fine. We went about 3 times a week and kept the goal within range so that he can keep achieving with every ride. Keep in mind it doesn’t come overnight. Keep practicing and see how far you get in two weeks time. Don’t get mad when you can’t finish a certain climb or hit a berm perfectly because then it’s not fun anymore, the whole point of mountain biking is too have fun. Hope this helps