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 record demand for Boston slots has much to do with the exploding popularity of marathons in the U.S.: The 10% growth in participation last year was the largest spurt in 25 years. The number of runners who qualify for Boston now far exceeds the available places (excluding about 5,000 spots reserved for charity runners).

But there's another possible reason for the surging demand—one that has the potential to kick up a fair amount of controversy. It's the notion that the qualifying standards for women are too soft.

By all accounts, the running boom is being fueled by women more than men. Women made up 42% of finishers in the 2010 Boston race—a proportion that is higher than the percentage of all U.S. marathoners who are women. But according to gender rules instituted in 1977, the marathon times women need to post to qualify for Boston are 30 minutes slower than the times the men in the same age group have to run. The problem: There's no evidence that women really need that much extra time.