The dragon was panting from exertion.
Teryx wasn't exhausted from the running, but rather, from trying to keep his arousal in check.
The marathon had too many other athletic males in tight clothing around him, and it was impossible to keep them out of sight by outrunning them all.

Commission from Tos_Tos_F_ski
https://skeb.jp/@Tos_Tos_F_ski/works/200