Made this same trip back in 1999 and loved every minute!
Camaraderie gave you excellent advice. I'll add just a bit to his comments.
From Norfolk you have the opportunity to follow the ICW east through the Virginia Cut or take the westerly route through the Dismal Swamp Canal down to Elizabeth City. We chose this route. From EC we followed the ICW all the way down to Miami.
From Miami you have 2 choices. Head south down the Hawk Channel to Marathon, then pass under the 7-Mile Bridge. You can also stay on the ICW, head southwesterly down Biscayne Bay, and follow Jewfish Creek over into Lake Surprise and the Gulf.
I've taken both routes and have used the weather forecast to help me decide which way would be best. I personally prefer the westerly route down the Keys; less traveled, more anchorages, and fewer crab pots to dodge. Also, if a stiff E or NE wind is blowing (common during the fall and winter), you'll appreciate how quiet the waters are on the west side. I draw 4.5 ft and had no problems either way.
Anyway, you've got a great trip to plan. Get good charts, waterway guides, and have a blast!