They do run early enough. However I get super anxious if I think we won't be somewhere "early" rather than just on time. We tried waiting for a bus once and I completely freaked out, got a migraine, and it sort of ruined my morning. We did actually make to our reservation on time, but I felt very stressed and rushed.
The second time we had a pre-park opening ADR, we just got a cab. We waited about 30 seconds before the cab drove up and picked us up and it cost about $15. That was from POR to AK. I plan on getting a cab to AK this year as well for our ADR. It was much more enjoyable for me to not be stressed. Most other people would be fine waiting for the bus, I'm just a bit nuts
So if you do want to get the bus in the morning I recommend you do the following:
1.) The night before, ask CMs at front desk of resort what time the buses will start running.
2.) Tell them where and when your ADR is
3.) Ask them what time they recommend you get to the bus stop
Then just make sure you leave plenty of time to get to the resort in case you do have to wait a while for a bus. A lot of resorts have the bus wait times listed on a screen now, which is very helpful.
Koda21, which resort are you staying at? I'll let you know approx. how long it takes from resort to park.