@samdell: I sympathize, as I'm dealing with a little bit of what you're talking about myself, though it sounds like you might be worse off, and just a few steps away from being a hermit!
The best advice that is working for me is maintaining the pendulum mindset. Right now, our pendulum is pretty far to the one side where we are being almost anti-social. On the other side, is the social genius who knows and is loved by everyone. In order to push that pendulum toward the other side, it's going to take a lot of extreme force. So, you need put forth some extreme effort in socializing and developing interests.
One thing that's helped me was taking "continuing education" classes at the community college where I live at. Make sure it's something social though. It doesn't have to certify you in anything, it can being something like beginner's golf, cooking, rock-climbing, whatever. Taking martial arts, as was suggested earlier, is also an option. Double benefit of being social AND gets you into shape. Museums, free/low-cost classical concerts at your local uni, photography, horse-back riding, astronomy...there's a whole world out there! Also, try MeetUp.com, a social site for finding groups in your area with particular interests.
Another thing to consider is that of depression. If you lack the DESIRE to go out and try new things, you could be depressed and not know it. That's well out of the realm of this forum though....consult a professional if you have doubts.