What a marathon runner does, but rarely wears? Crossword Clue

 The answer to the crossword clue "what a marathon runner does, but rarely wears" is TRAIN.

A marathon runner trains by running long distances. However, they rarely wear a train, which is a long, narrow vehicle that runs on rails.

The answer is a play on words. The word "train" can refer to both the act of training and the vehicle. In the context of the crossword clue, the word "train" is referring to the act of training.

I hope this helps!