These runners are competing in a short distance race, as is evident from the position of their arms. The race is probably the stade (cf. stadium) race, approximately 600 feet long. The stade race was both an individual event and one of the five events in the pentathlon (= 'five contest event').
The discus-throw was not an individual event,
but part of the pentathlon (along with the stade race, javelin-throw, broad
jump, and wrestling). Note that Greek athletes competed naked.