If you're interested in running next year's Boston Marathon, you'd better get set.

Race officials say the marathon's 21,000 prized slots, which used to take six months to distribute, could be filled in a matter of days after registration opens on Monday. In fact, every spot could be taken before the ING New York City Marathon—the nation's largest—takes place on Nov. 7. If that happens, New York's race would effectively be eliminated for the first time as a qualifier for next year's Boston.

The...