Setting my marathon goal finishing time

With this being my first marathon, of course my main goal is just to finish. :) But I am starting to wonder what kind of finishing time I should expect.

I've done a few half marathons with my most recent one being the SF half marathon where I finished in 2:08:13, 9:47 min/mi pace. That course was predominantly flat.

So I've been asking some of the other runners from my running group about this, and I've heard a couple of theories:

• 1) Double your half marathon time and add 20%: For me, this would be 4:42:05, 10:46 min/mi pace
  
• 2) Double your half marathon time and add 20-30 minutes: Using the 20 minute guideline, this would give me 4:36:26, 10:33 min/mi pace.

I think if I finish under 5 hours, I would be happy. :)

BUT here are some stretch goals for my own amusement:

• 4:14:54, 9:44 min/mi: P.Diddy in the 2003 NYC Marathon
  
• 4:29:15, 10:17 min/mi: Oprah Winfrey in the 1994 Marine Corp Marathon