I have driven and taken transportation, and both are easy. Buses will usually be waiting and leave right at the time indicated in the event guide. They pretty much run continuously before and after the race during the available window. Note that if you are a faster runner in an earlier corral, you may have to wait for the course to clear for return buses to start up again. Buses drop off a little closer to security than the parking area, and they pick up right behind the reunion zone past the finish line. Very convenient.
If you drive, there will be maps in the guide and well-signed roadways to guide you around any closures and detours. The parking gates aren't manned but CMs will be there directing you to the open parking area. You'll need to walk all the way up to the Epcot entrance to go through security, then back to the staging area. And this year, after the race, they had us walk pretty far up toward Epcot before we could double back to parking. So just know it's a bit more walking when you drive than taking a bus. You will want to leave yourself plenty of time -- I usually leave when the first buses start -- but you can hang out in the car and eat, nap, stay warm/dry, etc before time to make the journey to the staging area.
Barring any issues (like a huge accident causing road closures during the marathon this year), getting out is not usually difficult at all.