1. New York City Marathon 
47,000 registered participants in 2012 

The New York City Marathon isn't only the largest marathon in the U.S. – it's the largest in the world. Although the 2012 race was eventually canceled in the wake of Hurricane Sandy, roughly 47,000 people had registered to run. In 2011, there were 47,133 finishers – the race's largest number ever. The course travels through the five boroughs of New York City, beginning on Staten Island and ending in Central Park. It has been held every year (with the exception of 2012) since 1970. Because of the popularity of the race, registration is offered through a lottery system.  

The 2013 New York City Marathon is scheduled for November 3rd. 2012 participants will be given preference when registration opens. 

2. Chicago Marathon 
37,455 finishers in 2012 

The Chicago Marathon was founded in 1977, and has been held every year since then – with the exception of 1987 when only a half-marathon was held. There are no qualifications required to register, but participants must be able to finish the course in under 6 hours, 30 minutes. And approximate 1.5 million spectators come out to watch the race, which begins and ends in Chicago's Grant Park.  

The 2013 Chicago Marathon is scheduled for October 13th. Although registration for this year's event has already opened, it has since been suspended due to technical complications. An announcement will be made at 3:00 pm CST on Thursday, February 28th which will explain how and when registration will be reopened to fill the remaining 15,000 entries. See here for details. 

3. Honolulu Marathon 

24,413 finishers in 2012 

The Honolulu Marathon takes place annually on the 2nd Sunday in December. The race is considered moderately difficult given the relative hilliness of the course compared to other marathons. The tropical weather (typically ranging from 65 - 80 degrees Fahrenheit) is an additional challenge. There are no qualifying times to meet in order to enter, no time limit to finish, and no limit on the number of runners, making this a popular race for first time marathoners. (The picturesque location probably doesn't hurt either.) 

This year's Honolulu Marathon is scheduled for December 8th, 2013. Registration is not yet open.  

4. Marine Corps Marathon 
23,515 finishers in 2012 

The Marine Corp Marathon takes place in Washington, D.C. and is the largest marathon in the world that doesn't offer prize money, earning it the moniker "The People's Marathon." The race was initially founded 1976 as a way to improve the public's perception of the military services, which suffered after the Viet Nam War. There is no qualifying time required in order to register, but all runners must maintain at least a 14-minute mile pace.  

The 38th MCM will take place on October 27, 2013. Registration for the event opens on March 27, 2013 (active duty registration is open now). Participation is limited to 30,000 entrants. 

5. Boston Marathon 
21,544 finishers in 2012 

The Boston Marathon is the world's oldest annual marathon. It was established in 1897, making this the races 117th year! In 1971, in an effort to limit the number of entries to the popular race, the Boston Marathon began requiring that participants be able to finish the race in less than 3 hours, 30 minutes. The qualifying times have since been tightened to 3 hours, 5 minutes for males (ages 18-34) and 3 hours, 30 minutes for women (ages 18-34). (Qualifying times increased as age increases.) In addition, race registration is opened up to those with the fastest qualifying times for their age group first – meaning that only the fastest are able to register prior to the race selling out, keeping the Boston Marathon one of the most prestigious races in the world.  

The 117th Boston Marathon will take place on April 15, 2013. Registration is closed. Registration for the 2014 race will begin in September of 2013.  

6. LA Marathon 
18,729 finishers in 2012 

The LA Marathon is held each Spring in Los Angeles. It was established in 1986 and has been held every year since then. There is no qualifying time required to participate, but participants must be able to complete the course in eight hours. The "Stadium to the sea" course begins at Dodger Stadium and finishes at the Santa Monica Pier, with a net elevation drop of 430 feet. 

This year's LA Marathon takes place on St. Patrick's Day: March 17th, 2013. Registration is open, with limited spots remaining.  

7. Walt Disney World Marathon 
13,467 finishers in 2012 

The Walt Disney World Marathon claims to provide runners with "the most magical 26.2 miles on Earth." The course runs through all four Walt Disney World Theme Parks and includes on-course Disney entertainment.  

The 2013 Walt Disney World Marathon already took place in January. Next year's marathon will be held on January 12, 2014 with registration opening on April 9th, 2013. 

8. Philadelphia Marathon 
11,553 finishers in 2012 

Always a large event, the 2012 Philadelphia Marathon swelled to 11,553 finishers thanks, in part, to an extra 3000 spots that were opened for runners who had registered for the canceled New York City Marathon. The race is typically held on the third Sunday of November, and follows a scenic course that winds past several historic landmarks. 2013 marks the Philadelphia Marathon's 20th anniversary. 

This year's Philadelphia Marathon will take place on the second Sunday in November (November 17th, 2013) rather than the traditional third Sunday. Registration opens on April 1, 2013.  

9. Twin Cities Marathon 
8781 finishers in 2012 

The Twin Cities Marathon is an annual event taking place in the Minneapolis-Saint Paul area and claims to be "the most beautiful urban marathon in America." The course winds around four lakes and the Mississippi River and is lined with entertainment in the form of cheerleaders and rock bands. There is a 6-hour time limit for the race.  

The 2013 Twin Cities Marathon takes place on October 6, 2013. Registration is now open.  

10. Houston Marathon 
7637 finishers in 2012 

The Houston Marathon is held every January. The course is fast and flat (it's even been called the "fastest winter marathon") and lined with entertainment along its scenic single loop. The average temperature in Houston in January ranges from 45F (low) to 63F (high), making this a comfortable, as well as fast, winter marathon. There is a 6-hour time limit to complete the course.  

The 2013 Houston Marathon took place on January 13th. Next year's race will take place on January 19th, 2014; registration begins in May, 2013.