Slow down, you wild and eager woman. Halfway around a block is also halfway around a block back (unless you got a ride back somehow). Walk a little. Walk a little more the next day, if that feels right. If your knees protest, give them a day of rest and healing, then try the shorter walk again.
I love walking, and feel walking has been really good for my new knees. It's a great natural exercise, and what the knee joint is constructed--muscles, tendons, and the whole package--to do. Remember your muscles were moved around in surgery, and you haven't been using them correctly for a long time, most likely. Also, after a week or two of lounging around doing not much walking at all, those muscles are a little out of shape. So break them in slowly and work up to what you really want them to do. Me... I want to walk for miles. Currently I'm doing about a mile and change at a time. Not much, right? But when I started right after surgery, I could only walk to the bathroom!