What are the best endurance supplements for runners for that “extra edge”?

I’m in the Marine Corps and I’m going to have a PFT or Physical Fitness test in which I am evaluated on my pull-ups, crunches, and three mile run. I performed well on my last PFT and had about a 22:00 run time. I would like to get my run in the 20:30 to 21:00 range and I was wondering if anyone has experimented with anything on the market that could help boost my performance.

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