"There’s a guy (21 years old) in my run club that has improved significantly in the last 9 months to the point that multiple people in the club have become suspicious of his recent training."
"While at first inspired by his recent dedication and improvement, his fitness has grown so rapidly I, and several others have began to question the legitimacy of his recent success."(quote)
This is the part of the original post that most commenting here have overlooked. The OP isn't the only runner in his club who finds the improvements suspicious. Indeed he says "multiple" other runners in the club find it so. So they are all clueless? Or are they quite plausibly seeing something that isn't explained by their perceptions of the runner in question? It is apparent that most commenting here would be amongst those who observe nothing out of the usual. Since that is so with the sport generally I am not surprised.