I want to take up something but my crush...?

I want to take up athletics running at school again. I used to be a really fast runner at twelve, i'm now fifteen and a great deal unfit, so i don't want to embarr myself and be the slowest one running. Also, a guy in my year who i had an embarring thing/rejected with will be there and i don't want to look stupid but i really want to do it for confidence? Idk what to do.