Tony, the state champion cross-country runner, often appeared

or tireless, to his competitors.

august

ancillary

asinine

indefatigable

The right option could be:

indefatigable.

Explanation:

To be a very good athlete, you regularly need a very good physique and endurance, which is why, in a sport as demanding as cross-country, to be the state champion, it requires an excellent resistance that allows you to reach the goal in the shortest possible time, likewise, for his competitors, who surely have not yet overcome him, it must seem to them that Tony is someone indefatigable, in turn, this word is synonymous with tireless, a word that is found below in the text.