old mule

Mules are not wild animals. They are the product of breeing horses and asses. They are legendarily stubborn and self-willed. William Faulkner described a mule as an animal who will work patiently for your for twenty years just to have the opportunity to kick you once.

Old mules are proverbially mean and liable to kick anyone who is attempting to handle them.

so, the guy compares himself to a hard-working surfer and he's happy he did his best even if he lost.

am i right when i'm saying that "to go out" means to be eliminated?

Since i don't know the context, how could i say? If you are referring to a competition, then you are probably correct.

EDIT: Sorry, i see that you did refer to a competition. Yes, that is likely the meaning of "to go out."

thank you:)