During the final miles of the marathon, his pace slowed as he began to tire.

tire 是动词，意思是"感到疲劳、疲倦"。这个词可以指身体上的疲惫，也可以指精神上的倦怠。在马拉松比赛中，运动员会因为长时间跑步而体力消耗，自然就会开始感到疲劳(tire)。