DH and I walked the bridge 3-4 years ago. We spent the night in Gaylord (Red Roof Inn) then got up early and drove the rest of the way. Had to park several blocks from the starting point, but that was no biggie. Walked to the Traverse Bay Woolen Co. and they had shuttle busses there to take people over. We had to wait in line awhile for a bus. They took us over, dropped us off and we walked back, across the bridge. It was an awesome experience to be able to do that, you get a different perspective of the bridge than when you drive over it. Really neat and fun and I would love to do it again some time.
Many, many people walk the bridge so if you want to spend the night in Mackinaw City or St. Ignace I would recommend calling to make a reservation. There are many hotels but I bet they fill up for that weekend.