It is 20 seconds or less! I cheated a little bit with the "distance" markers. No matter how fast you run, the first 75% of the race, (112.5 meters,) lasts exactly 15 seconds. During the first 15 seconds, the "distance" markers are just measuring time. If you intentionally run slowly, you will notice how they appear closer together to compensate for your slow rate of movement. (Not reccomended, since there are reports that this breaks the game!)
Once you arrive at the 15 second mark, the finish line is dynamically selected by multiplying your slowest possible speed by 5 seconds. So you will never finish in more than 20 seconds, but a winning race will probably be closer to 17 or 18 seconds.
The result is a footrace where all the competitors sprint for 15 seconds, and then the finish line is drawn 37.5 meters ahead of a particular runner. A little wacky, but it respects the distance gained or lost by individual runners while still fitting the jam constraint.
Great question, and thanks for playing! :)