Negligible.
It's a fairly quiet time at the parks - I'm sure Disney planned it that way. I've been there once as a spectator and four times as a runner.
Many participants come for a long weekend and go... packet pick-up on Friday, family fun run Saturday morning, marathon and half on Sunday. For most of the crowd, hitting the parks is part of the weekend, but not the main event. The leukemia "team in training" runners are allowed to stay only a limited number of days, so they clear out shortly after the Sunday race.
Specially hired buses are brought in by Disney to transport runners and their families to the various events, so there is no strain whatsoever on the normal WDW system. On race day, the last buses to the starting line near EPCOT leave the resorts at 4:00 am. Spectators who choose to follow the course may hop the monorail to MK, but since the race starts at 6:00 am, it's too early to interfere with the normal Sunday park crowd.
By the time the parks are open, the runners are very spread out over the 26.2 mile course. There are cones set up and CMs posted to keep the course clear through AK, MGM and EPCOT. The park crowds are fairly sparse anyway, so it doesn't seem particularly disruptive. Most people cheer, though I once heard a guy grouse because he had to wait for a few runners to go by before he could cross the street.
After the race, runners tend to crash in their hotels, so don't expect them to crowd the parks (though the hot tubs might be a different story). For the next few days, there's a lot of limping and wincing - not much competition for the park commandos.