Yes. Yes there is. People who run races stay in on-site hotels. They also go to the parks. They also spend crazy amounts of money just to enter the race (seen below). It is a huge market for WDW and one that has helped turn January from a slow season into a fairly busy period for most of the month now.
The WDW Marathon is also run by about 26,000 runners each year, so no, this is not similar to Golf. Those 26,000 also aren't counting any family or friends who come down with them. They also spend quite a bit of money.
This is current pricing to run the WDW Marathon (one person):
Walt Disney World® Marathon
Dopey Challenge
Goofy's Race and a Half Challenge
Walt Disney World® Half Marathon
Walt Disney World® 10K
Walt Disney World® 5K
Race Registration | Walt Disney World Marathon | runDisney