I've done several marathons and a few ultras. Once you get to the furthest run, my opinion is that it becomes very personal and hard to take advice. With that said, I'd aim more for time on feet than distance.
So, for this type event, and not meant for a well trained athlete but a "first timer" type I'd recommend maybe a 3-4 day consecutive Dopey type training event of short run, 1 hour run, 2 hour run, 3-4 hour run. Then let the pace fall where it may. I'd do this 5-6 weeks out. Tailor to fit your needs.
I'd also recommend looking at this not as a marathon, but as a 48 mile run with another 20-40 mils of walking. Think of all the mileage you'll be putting gin and prepare for the entire Week starting with the Expo, the runs, and doing whatever walking of the parks you have planned.