Saturday is 1/10, Sunday is 1/11 - just FYI.
First thing to note, the marathon course for 2026 is not out yet, it won’t be published until closer to the event. Granted the course can change, but in recent years access to the All Stars/World Drive/Osceola Parkway on that “south” end of WDW has not been impacted. The closest it has gotten is just north of there at the BB parking lot and Western Way.
As the PP said, Disney puts out a detailed road closure map in the guide that’s released a few weeks before the event. I usually post it here on the Resorts board for reference.
Personally, while there are some road closures and a few detours needed around property that day, I wouldn’t personally change any plans. Disney’s strong bias in recent years has been to get runD disruptions out of the way as fast as possible (the main reason the marathon start was moved from 5 to 4:30 a few years ago) in favor of regular guests. Again, the course can change (there are some questions as to how AKL or Western Way construction may impact things for 2026) and there can certainly be some pinch points around property that linger into daytime hours given the length of the event, but it’s not all out chaos around and nothing is completely inaccessible.
I’d keep an eye out on the course and road closures map just to be prepared, but otherwise not worry too much about it.
Not sure which mini golf facility you plan to go to, but in recent years the course has gone through the BB parking lot. BB has typically been closed for the day, I’m not sure if Winter Summerland has also been closed or has adjusted hours that day, I’ve never really looked. You could hit some traffic/detours heading to Fantasia Gardens as typically eastbound E Buena Vista drive is closed for runners as they turn into DHS. You can still get to Fantasia Gardens you just have to get to E Buena Vista drive from the east heading west.
Hope that helps.