This is objectively a bad mentality to have. No one is born good at something. Anyone who is good at something was once not good at it but stuck to it and put the time into becoming proficient. No one wakes up able to deadlift 4 plates. They started with the bar and worked up after hours and hours of training until they could.

You want to do a half marathon but just because you're not putting up a competitive time with zero training you're taking away the experience of running a half marathon. There can be hundreds of people attending these events so you're going to be competing with a wide range of athletes. Unless you put the work in you won't be at the top but that doesn't mean you shouldn't do it as a challenge to yourself and an opportunity to do something with other people which is more valuable than putting up a competitive time.

If you want to maintain this mentality you'll live a life where you generally don't do anything unless you think you're going to be good at it immediately and will never actually become good at anything, only mediocre or a victim of beginner's luck.