I agree with Susan that maintaining a steady state for the entire distance can be a tad fatiguing because when we run outdoors we tend to vary our speed. Not to mention if you used to running on a hard surface, moving to a softer surface of a treadmill can make a difference, too.
Lastly, racing is not the same as training. You do kick it up a notch, after all that is what you spent months training to do.
RUN SPARK STRONG!