The marathon distance of 26.2 miles was established at the 1908 Olympics in London. The marathon course was designed so that it could start at Windsor Castle and finish at the Olympic stadium. The distance was extended to accommodate the British royal family, as Queen Alexandra requested that the race start on the lawn of Windsor Castle and finish in front of the royal box at the Olympic stadium, which happened to be 26.2 miles (26 miles and 385 yards). In 1921, the length for a marathon was formally standardized at 26.2 miles (42.195 kilometers).

So no, it is not a round number in the metric system. It has more to do with British royal family preferences than anything else!
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