I'm going to call myself a marathoner or say that I completed a marathon
Have you completed a marathon?
I would have finished it if I don't have an injury(That's a lie).
It is okay to want to be admired but lying about your accomplishments is wrong. People will not trust you at all.
So what if they don't? I don't care is they trust me.
People are better at detecting lies than one would imagine. Lying about achievements can often backfire, and once the lie is found out, you'll lose people's trust and embarrass yourself. Once this happens, they're less likely to spend time with you, do things with you, and believe you.