Saturday is probably better, but only marginally. WDW does a great job organizing the races, including the impact to the parks. Most of the races are early enough that they do not impact the parks at all, but the marathon had runners in the AK, HS, and EP areas well into the early afternoon (we ate at Beaches and Cream after I finished then went out to cheer on the runners).
As for crowds, it honestly was the most relaxed trip to WDW we've had. Granted, it was just DW and myself and we can tour pretty efficiently, but I ran races each day and then we'd go to the parks and crowds weren't bad at all, IMO.