She hit a wall during the marathon and could barely keep running.

wall 在这里不是指字面意义上的"墙"，而是比喻"障碍、极限"。 hit a wall 是英语习语，字面意思是"撞到墙"，比喻"遇到难以克服的障碍或极限"，特别指在长跑等耐力运动中突然感到极度疲劳的状态。