Hurt is like the word 痛 or 疼 or 伤 You would use "it hurts" in response to a question like: 哪里痛？ Where does "it hurt" ? 我头疼 My head hurts. In your example sentence: "It hurts at first, but then you hit a certain point, I feel like I could go forever." This sentence might be talking about running (赛跑) or biking ( 骑自行车) where "it hurts" because it is difficult (难) at first but over time you learn and get better (学会, 学习) and it becomes easy (简单).

You can also say: It hurts me when you tell me lies. It hurts people when others bully/attack them. It hurts his feelings when his girlfriend cheats on him.

When you feel pain, you say: "it hurts".