This mental fortitude was probably because he had stopped studying and started helping his family out at home. He was often exposed to the sun, which then helped to improve his physical health.
He managed to go from barely being able to sit in the horse stance for more than 5 minutes to comfortably sitting in the horse stance for one hour! This was a huge improvement.
Even if he wasn't very naturally talented in this area, at the very least, his aptitude was considered top-notch.
In addition, Xue Yan was an extremely smart person. In terms of learning, he could quickly pick up patterns and diligently mimic them. As time passes, Xue Yan's skills will improve and he can be trained to a very high level.
However, Xue Yan knew that he would never be able to reach Jiang Yue's level.
After all, she had the space that supported her. Moreover, she had all kinds of abilities that had evolved in the apocalypse in her past life.