If you're planning to register for the 2016 Boston Marathon between now and Wednesday evening, your chances of actually toeing the line on April 18 are dwindling.

Approximately 5,000 spots remain after the first week of registration opened last week for the fastest qualifiers, the Boston Athletic Association told Runner's World on Sunday. That number is about 3,000 less than last year at this time for the 2015 race.

As Boston hopefuls have found recently, simply qualifing for the marathon doesn't guarantee entry anymore. For the last two years, the number of applicants has exceeded the number of spots alloted for time qualifiers, and the registration system is set up to allow entry first to the athletes who run faster than their qualifying time by the greatest margin.

Last week on Monday, three consecutive periods for registration were held—those who ran 20:00 or faster than their qualifying time could register starting on Monday, followed by those with times 10:00 or faster on Wednesday, and runners with times 5:00 or faster on Friday. About 19,000 runners signed up.

This morning registration opened for all time qualifiers, regardless of the times they ran to meet their standards. Registration will stay open until Wednesday, September 23, at 5 p.m. Eastern.

If the number of applicants exceeds the approximately 5,000 spots available after Wednesday evening, however, runners will earn a bib based on the amount of time by which they ran under their standard. The fastest runners will gain entry first until the field fills.

In 2014, nearly 10,000 runners tried to sign up for the approximately 8,000 remaining spots. Runners ultimately had to notch times at least 1:02 or faster to make it into the race.

From a total field of 30,000 runners, about 24,000 participants will be time qualifiers. The rest of the field will consist mainly of runners participating for charities. These numbers mirror those for the 2015 marathon.

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