My friend and work colleague did the London Marathon in 2007 and she found a running partner and they ran outside, building up their distance gradually.
They spent at least 6 months building up to the Marathon training. So They began with 3 miles, then 6, 9, 12 and so on. The 3-4 days before the event, they didn't run at all, just did some cardio, low impact in the gym.
I'm sure you know this, but don't wear running shoes that haven't been well broken in. If you're a woman, then vaseline on the nipples and a good sports bra is a must.