Like above said - a rental car will be less expensive then the shuttle! $75 per person plus tips. And that would be to the middle Keys, you still have to get to KW.
From Postcard Inn - there isn't much to walk to except Theater of the Sea. There are some restaurants withing walking distance - but I can't recommend the closest ones at all. (Never been to Shula so can't say but check the reviews) The Whale Harbor restaurants - go for cocktails on the bridge bar - NOT for food. Ocean View, across the street is good. A little farther down is Island Grill and Smugglers Cove - both are good. Other then that you aren't going to see much on foot from that location.
Islamorada is a long chain of islands. The better area (if you're still going the foot route) would be around MM 81 - 82ish. Close to the same price as Postcard - Give Casa Morada a look, as well as Sands of Islamorada, Pines and Palms and Kon Tiki.
But, reconsider the rental car one way - turn it in in KW. I've been able to get a car for less then $30 a day one way.