We had Tusker at 8:00 and we had family meeting there from several resorts. Two resort front desks told folks buses don't start until 8 and they would need to take a cab. I was texting that night to reassure them but one ended up cabbing it anyway.
I knew the bus would be there - we walked down at 7:15, bus was there within a couple minutes, we were at AK with plenty of time. Anyway, while we were in the bus we were texting the family to confirm "buses are running guys!" and when we stopped at AK the bus driver said he overheard us and to "never get transportation info from anyone other than a bus driver!" and he confirmed buses started at 6:30 for that time of year.
I would leave earlier for MK since its a little further, but you WILL have a bus!