As for Charleston, SC and most of the south for that matter, there isn't much focus on walkability. And as noted, if you wanted to walk around, it would be downtown in the historic district, and that is mobbed with tourists.
Weather is nice, hurricanes are not too often, and you can always go to the mountains if you get tired of the beach.
If I was moving south from some cold climate, I'd probably look at Greenville SC the hardest. It is relatively cheap compared to other retirement destinations, has a decent downtown area, and a surprisingly frisky arts community.
But we'd still love to have you in Chucktown, just be aware of the need to mosey on up the highway every now and again during hurricane season.