I can second this. (I used to live on the UP about 300 years ago) Actually....either direction on the Bridge can be an adventure. Most especially on a bike as much of the road surface is of the see-through grating variety that can be difficult for some people to navigate on two wheels. Weather can be an issue as well. However, as you're planning a summer crossing, you should be OK. If you should cross on a blustery day (and there are often many on the Great Lakes).....go SLOOOW.
Not sure that I'd go along with a week being too much. It depends upon why you're there. If you're looking for night life (other than the wildlife sort) or large cities or towns.....then yes, you'll be disappointed and bored. However, if you're seeking to get away from people and camp in the woods, do some fishing, etc.....then you may never want to leave. (I didn't)
If you want some touristy sorts of things that include other people...Mackinaw City is worth the time (still no nightlife though....more of an old-school resort) If you've seen the Christopher Reeve/Jane Seymour movie 'Somewhere In Time', it was filmed there. (or much of it anyway)
As with all things...the UP is solely what you make of it. It's one of the last unspoiled areas left in this country. And that won't last much longer. 'Leave nothing but footprints and take nothing but pictures.'
.....sT