We always drive I-75. It usually takes 24-25 hours depending on road construction and traffic. I am a huge fan of the book "Along I-75" by Dave Hunter. It is available at CAA, Chapters and on the internet. The book (updated annually) lists every exit with details about restaurants, gas, and accommodations. We usually stop somewhere around Knoxville, Tennessee the first night going down. Coming back we usually stay around Corbin, Kentucky.