Michigan is pretty devoid of decent climbing. You can get OK rollers up in the Traverse City / Petoskey area, out onto the peninsula, but in the southern tier, it's not climbing unless you're on a MTB.
To get real climbs, you're looking at 4-6 hours drive in any given direction:
- South, to Southern Ohio toward the KY/WVA borders - that's where they run the TTT,
- South, to Bloomington, IN has a great tri community and some decent hills,
- West, to Wisconsin where they run IMWI and the HHH century, or
- East
(my favorite
), to the NY Finger Lakes. Awesome riding, rides, wine country, and beaches.
Someone from PA might be able to weigh in on the riding around Pittsburgh; it's hilly enough, I just don't know if it's cycling friendly. That may be a good option. Also the U.P., don't know much about riding up there, but I know there's a hill. Just don't know if there are roads.
Once you extend your drive to 8 hours you're bringing Kentucky, TN, WVA, more of NY, and Central PA into play, which all have hilly riding and great roads.