For Marathon Weekend...on Saturday, begin your day at DHS and/or AK, because the Half does not go through either of those parks. And really, even though the Half does go through MK, all the runners are through there before the park opens, so you shouldn't be affected by runners in the park.
On Sunday, I would start the day at MK, for the same reason listed above, and stay there all day, or if you want to move to another park, move over to AK, because that will be the next park to clear (probably by noon). Epcot is impacted the most on both days because it is the start and end point for both races.
Honestly, your biggest issue on Marathon Weekend will be transportation, due to all the road closures for the races. You will want to allow plenty of extra time to get to the parks, and it is highly recommended that you use Disney transportation to get around, because the buses can go places the cars can't and the traffic will be generally terrible around WDW for the first half of both days.
But in terms of crowds in the parks, they are relatively light, so it is still a good time to go.
I'm hoping it something I can use!
I would have preferred AKL club level at 40% off, instead of 25% off. 
It wouldn't be that much more to stay at AKL with the 25% off, over 40% off at WL. Decisions, decisions!
