I had this same question last March, but I didn't foresee this problem until the night before the breakfast. I got on the phone and they assured me that the buses start running in plenty of time to get there on time. We were at PO Riverside and made it to the MK right at 8 with reservations for 8:15. We have reservations at 8:20 in March, so I don't think it will be issue.
I got some great pictures of main street before the park opened, you can see them if you click the link in my signature.