Tryathlete

A person who thinks they can complete a triathlon without much training and fails miserably when they try.

That guy talked so much trash but he's only a tryathlete. He DNF'd (did not finish) by almost drowning during the swim.

Tryathlete

Someone who trys to be an athlete.

I'm a tryathlete, I always try to get to out of the house to train, but there are too many distractions in my fridge