1) Recover quickly from the marathon, or at least feel reasonably OK ... whatever that means to you.
2) Wait until after the marathon to make a decision about the half marathon.
3) Have a good understanding of your own body and how you recover from races.
4) Don't do this for a first-time marathon or half marathon.
5) Take extra rest and do a couple of short easy runs. Legs not sore? You're good to go.
6) Be prepared to take the half marathon easy.
7) Run with running buddies who you absolutely will not abandon.
8) Enjoy your race! Chat with other runners, interact with spectators, take pictures, linger at aid stations, and stop to walk whenever you please.
9) Do it for the T-shirt and medal.
10) Accept your certifiably insane label ;-)