Once the boat was drained I made my way back in without further mishap, but I had a BB sized hole to deal with, and the issue of how to keep it from happening again.
The place I bought the boat said they could weld a new piece of plastic over the original keel for about $60. It's good to know they can do it, but I really didn't want to spend that much money if I could help it.
I looked online and considered all kinds of patch kits, keel guards, etc., and eventually found something that has been working very well.
Go to a hardware store, and for about $4 you can get Marine Epoxy Putty. You mush the two parts together to activate the putty, then use it to patch the hole. It takes an hour to harden completely, but it will harden wet if need be.
I check the keel often to make sure the patch is still holding. It's been working fine for several months, and is just now beginning to chip a little around the edges. I only used about a quarter of the stick of putty on this job, so I'll just scrape off the old patch and put on a new one. I wrap up the remainder to keep in the kayak as an emergency repair kit. You never know.
The patch scrapes easily, too, and I knew I had to quit dragging my boat. I needed a kayak cart of some sort.
Once again I went online to see what good ideas I could steal...er, use, and on my second try came up with a pretty good solution:
How to Build a Kayak Cart
Any encounter with a rock, pavement, or even a sharp stick will leave a scratch. Dragging my empty boat over pavement always left a red trail...bits of the bottom of my keel. 
