Olympic silver medalist Keflezighi, 37, is the oldest American ever to qualify for the Olympic marathon. He trains at an altitude of around 9,000 feet in Mammoth to increase his red blood cells and boost his endurance.

London 2012: Oldest Marathon Runner Trains

U.S. marathon runner Meb Keflezighi poses during training for the London 2012 Olympics in Mammoth Lakes, California May 29, 2012. Olympic silver medalist Keflezighi, 37, is the oldest American ever to qualify for the Olympic marathon. He trains at an altitude of around 9,000 feet in Mammoth to increase his red blood cells and boost his endurance. Picture taken May 29, 2012. REUTERS/Lucy Nicholson

London 2012: Oldest Marathon Runner Trains

U.S. marathon runner Keflezighi trains for the London 2012 Olympics in Mammoth Lakes, California. Lucy Nicholson /REUTERS

London 2012: Oldest Marathon Runner Trains

U.S. marathon runner Keflezighi rubs his eyes after a training run for the London 2012 Olympics in Mammoth Lakes, California. Lucy Nicholson /REUTERS

London 2012: Oldest Marathon Runner Trains

U.S. marathon runner Keflezighi trains for the London 2012 Olympics in Mammoth Lakes, California. Lucy Nicholson /REUTERS

London 2012: Oldest Marathon Runner Trains

U.S. marathon runner Meb Keflezighi poses during training for the London 2012 Olympics in Mammoth Lakes, California. REUTERS/Lucy Nicholson

London 2012: Oldest Marathon Runner Trains

U.S. marathon runner Keflezighi trains for the London 2012 Olympics in Mammoth Lakes, California. Lucy Nicholson /REUTERS

London 2012: Oldest Marathon Runner Trains

U.S. marathon runner Keflezighi rolls up mat after stretching during training for the London 2012 Olympics in Mammoth Lakes, California. Lucy Nicholson /REUTERS

London 2012: Oldest Marathon Runner Trains

U.S. marathon runner Keflezighi poses during training for the London 2012 Olympics in Mammoth Lakes, California. Lucy Nicholson /REUTERS

London 2012: Oldest Marathon Runner Trains

U.S. marathon runner Keflezighi trains for the London 2012 Olympics in Mammoth Lakes, California. Lucy Nicholson /REUTERS

London 2012: Oldest Marathon Runner Trains

U.S. marathon runner Keflezighi trains for the London 2012 Olympics in Mammoth Lakes, California. Lucy Nicholson /REUTERS

London 2012: Oldest Marathon Runner Trains

U.S. marathon runner Keflezighi stretches during training for the London 2012 Olympics in Mammoth Lakes, California. Lucy Nicholson /REUTERS

London 2012: Oldest Marathon Runner Trains

U.S. marathon runner Keflezighi stretches during training for the London 2012 Olympics in Mammoth Lakes, California. Lucy Nicholson /REUTERS

London 2012: Oldest Marathon Runner Trains

U.S. marathon runner Keflezighi trains for the London 2012 Olympics in Mammoth Lakes, California. Lucy Nicholson /REUTERS