It mentions she is running a 5K race. That means the distance of the race is fixed and will not change. That means :
<u><em>Length of the race</em></u><u><em> is the </em></u><u><em>independent</em></u><u><em> variable and the </em></u><u><em>dependent </em></u><u><em>variable is </em></u><u><em>time</em></u><u><em>.</em></u>