I did the Detroit half a few years ago. It a GREAT venue! If you're afraid of bridges, then run to the left away from the edge. Although the Ambassador Bridge looks huge from the ground, it's really a very easy incline. You won't have a problem. The particular morning I ran it was clear and you can see downtown Detroit. We watched the sun coming up behind the Ren Cen, and a barge passing under the bridge blew its horn in support of the runners.
I, too, get a bit claustrophobic, and really didn't like the idea of the tunnel. However, it's well lit and there are a lot of people in there with you, but not packed so tight that you feel trapped. I had to laugh as people hooted as they entered the tunnel to hear their echo - like a bunch of kids. Very fun. Once you come out of tunnel, there is a crowd cheering everyone on. I felt like I was in a world class race!
Great choice for a marathon.
"God gave you your body as a gift, so you should take care of it." - My Mom