The races all start at 5:30 am. Courses are not established until about 6 weeks prior to the race, or later. 5K has been in AK parking lot and park ovewr the past few years, but done by roughly 7:30 or so. It is not a "timed" race so there are not any limits on time. Could be some parking issues or ingress/egress at AK. 10K this past year was started in MK parking lot and went to DHS and finished in EPCOT. many runners had to park at Epcot and bus to start area, but this is between 3:00 and 4:30 am, so little effect on any park goers. roadway for run is away from MK back toward DHS, so no impact on traffic coming into the parks, though some potential lane closures on a few roads. Runners should be through DHS by 7:30 am so no real effect in the park. Finish was through boardwalk to Epcot IG and then out front to parking lot. Some guests inside park were held at crossings for runners, but only in one or two areas. Some potential impact to Epcot parking lot entry, though it is staged on the far side and does not really effect those coming in other than volume. The half started in the MK parking lot too this year, but previous year was at WWOS, and before that was a night race. Timing on Sunday is slower for the longer race, with some lane closures but no full road closures. Course went from MK down back road to AK, so no impact to MK parking or ingress. From AK to DHS, but on the egress side of the road so no closures on the entry road to AK. Runners may not clear parking lot until 8:00 or so, but for both 10K and Half a 16 minute /mile pace is in effect so dawdlers will get swept off the course if they don't keep time. Runners through DHS to boardwalk and IG at Epcot, then into EPcot WS and through to the finish in parking lot. Runners in Epcot until 10:30 or so but only in coned areas. some lines waiting to cross may occur. Last year had around 20,000 runners in the half for perspective. from lots of other discussions on the Dis, parks have been extra crowded at times that were previously slow, so it is not easy to predict whether future crowds are going to change, but the actual race participants do not really change crowd levels. With the exception of Sunday night at Epcot when the race party occurs (party starts at 8:00 pm, and all "regular guests" leave after Illuminations anyhow).
If for example you like to walk the path between DHS and Boardwalk in the morning, it will be closed for the races, so that might be an inconvenience. If you like to hang out on your balcony and soak up the quiet on the Boardwalk or Y&B, you will have runners on both Sat and Sunday mornings. If you like to get to the parking lot at AK at 6:00 on Friday to hang out, it probably is not going to work for you that day. Otherwise, you will see minimal park impacts. As PP mentioned, it is possible for early bus issues, but they lease out buses from an outside company to provide the racers transport, so I'm not sure if that will or won't be an issue. As many have said before, bus transportation can be hit or miss at all resorts during all times of year.
Unrelated to the races, it is Halloween parties, F&W, just a few months removed from opening day at Toy Story Land, generally a good change in weather in Orlando, and the start of Christmas decorations and
MVMCP at that time, which might impact crowds.