Jon C wrote:

I guess I'm just trying to figure out how far
south we
have to be to provide a good day's journey, though nothing that would
wipe out
inexperienced, yet fit, 32 year olds.

I have no idea if I'm representative or not, but paddling one leg up
the Ohio River, and taking it fairly easy on the leg back, I can
easily do over twenty miles in five or so hours. I've gone 25 miles
in six hours, again, half against the current, and half with.

I'm extremely impressed with that. I recently kayaked around Manhattan, the
tide with us the whole way.. I believe the trip was just under 30 miles.
The 40 or so of us did it in just under 9 hours including a pair of 45
minute breaks.

Yes, I used to regularly run that trip for the MH Canoe Club starting in
NJ at Liberty State Park and going counter-clockwise around Manhattan.
AIRC, the actual distance was about 32 miles but the effective
'flat-water' distance was only a little over 20 miles due to the aiding
effect of the tide and the Hudson current.

I'd urge the original poster to be rather conservative in planning his
trip. The distances and speeds cited by most posters so far are
certainly believable, but seem on the high side for a pair of paddlers
who are inexperienced at kayaking - especially if there's the
possibility of significant headwinds.
The kayak/canoe trip around Manhattan was certainly more strenuous than
a typical 10 mile hike or 40 mile bike ride that the OP gave for
comparisons.