If this is your only day at MK, then I agree with pps that you need to change this ADR and take advantage of those early, low crowd, park touring hours. Eat a breakfast bar at the resort, and plan breakfast around 10:00.
Even if you have another day planned, if that day is not EMH, you will still miss that extra hour, but I might keep the ADR if I couldn't change it and it was important to someone in the family to have a meal with Pooh. If you decide to keep it, then you need to be at MK EARLY, EARLY and be one of the first people through the tunnel onto Main St and walking very fast. I would think you'll need to be no more than 5th in line to see A&E and still get to your ADR.