I would hang out at the porta-potties pre race. He's bound to go sometime before the gun goes off. Once he exits the porta-potty, barge your way into the same one and
1) if you smell fresh urine or
2) you see a steaming fresh pile of #2
promptly exit the porta-potty, run after him, introduce yourself and compliment him on 1) or 2)
If there is no indication of either, run after him, firmly demand what he did in the porta-potty. If he gets nervous, pants him, and confirm that he is, in fact, an alien.
In either case, you should have a good race on hand when you come up behind him during the race, breathing on the back of his neck, and whisper "mmm Alan boy, it's me again, I like that sweet ass", proceeded by a slap and squeeze. Dig a few fingers into his crack, pull up beside him, look him straight in the eyes, hold those stink fingers under your nose and take a long, deep, nostril-flaring smell.
Course record guaranteed or your money back.