It's one of the great charms of the Boston Marathon that 20,000 ordinary athletes of all ages get to compete in the same race, on the same course, at the same time as the world's best runners. In theory, any of them could win. In reality, however, the winners will come from the ranks of the so-called "elites," world-class athletes who come to Boston to compete for fame and a share of the half million dollar prize purse.