Buses start running to the parks by 6:30am, each morning. They don't run quite as frequently as the regular park buses do, so plan accordingly. For an 8am ADR at Tusker House, I would be out at the AK bus stop no later than 7:00, a bit earlier would be good if you can manage it. You could face a wait of about 20+ mins, and then the ride to AK, about 8 mins, then through bag check and the front gates, and the walk to Tusker House. If you have to wait for a bus, then you'll need the whole hour. If you're able to get right on a bus, then you'll be early...not such a bad thing.