Snap, great job! And, you will not take 4 hours for your half. Your per minute mile time will come down.
Back in May I took a speed walking clinic that taught me how to walk fast! I can walk a 11 minute mile when I'm working it, or a 13 minute mile if I'm cruising.
With proper technique, and practice, you can get your time down. Have faith!
Here is a resource to check out on speed walking:
http://www.racewalk.com/Shop/RWLCDVDITechnique.asp
If you look at the links at the top, it gives step-by-step instructions on how to speed walk.
Good luck! You can do it!
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"