You are not going to make a 7am ADR if you are not within walking distance of that restaurant, without taking a cab. The buses start running by 6:30, every morning. But, they don't run nearly as frequently as the regular park buses do...so if you miss that first bus, you may not be on the next one until about 6:50ish.
Then, if you are able to get on the first bus, at 6:30, you still need to get to your destination and then to the restuarant. If that's at a resort, then you have to plan on heading to a park first, and then to the resort.
You really are better off just taking a cab. That way, you can leave your resort about 20 mins prior to your ADR time and go there directly.
Let me give you a few examples. Let's say you're staying at Boardwalk. You have an ADR at 'Ohana at 7am. You would need to get the MK bus by 6:30, then you might be lucky and the driver would offer to drop you at the Polynesian....or, more likely, you will get to the MK by 6:50 (your bus will be stopping at the S/D and probably the YC/BC also), then you have to wait until the monorail is up and running (at 7ish) and get to the Polynesian that way....about a 15 min ride.
Or you want to go to Chef Mickey's...you may be able to get dropped off there, but if you aren't, you would have to walk over from the MK....just under 10 mins.