Now this should be done at your own risk, but it always worked for me. I was a rower for 11 years and as any rower knows, blisters on the hands are par for the course. Whenever I had a particularly troubling blister I lanced it, drained it, cleaned it with peroxide and then kept it clean. Then for competition, the best thing to do is cover it with New-Skin which is availible at any drugstore. Be sure to give it a healthy coating with this stuff.
Warning: New-Skin burns but once you're past the initial burning (which is an antiseptic at work, which is good after all...) it hardens and is very comfortable and protective.

ENJOY!

Yea I have new skin....I just doubt it will last the entire 13 miles......im prepared to have a red sock by the end of the race though lol

You'd be amazed at the durability of new skin, just make sure your skin is clean and well prepped before you apply it, also, Like Santiago says, make sure you have new, good socks on.

I am not a big fan of draining a blister because as a medic we are taught not to do that due to infection. However, as an athlete there is no way you can compete or continue to train with a blister or hot spot. So as an athlete, I agree with everyone on this thread. Don't cut it all the way open, just enough to let it drain the flap of skin acts as a bangage, a lot of the pain is releaved that way. Then make sure you keep it clean and cover it so it wont get any germs inside. Keeping it covered and clean should allow it to heal without infection.

Ya beat me to it VTIRON. Yes duct tape works great, for BNS and for blisters:D Just use a piece over the blister after you pop the blood or fluid out using a pin. Insert along the side and drain, then duct tape it.