Runners frequently attempt to distract themselves from the effort of running. We count the number of other runners, ponder work problems, and recite lyrics. In fact a study in The International Journal of Obesity used music as a form of distraction for obese children and adolescents. They found obese participants ran longer when distracted by music than when they had no music. Presumably distracted participants found the running less effortful, and so they ran further.
For the male runners in the present study, a female onlooker could be an excellent source of distraction, resulting in less perceived exertion. Male onlookers could have the opposite effect. As a male onlooker is a more typical competitor of the male runner, the male runner might focus on running and frequently evaluate the effort that it takes. The consistent focus on effort with a male onlooker would lead the runner to perceive the run as relatively effortful.