Knowing that I don't have any experience in running longer than marathon distance, I need to absorb a lot of wisdom from ultrarunners. I'm starting to read about how people prepare for a run of this distance, and what they do during the run to finish.
I've found a lot of material online. One page I'm reading is http://www.ultrunr.com/train100.html.
The main thing I need to consider is when my last triathlon will be this summer/fall. Training for a 24 hour run is different than for a triathlon. The main difference is the long runs that go from <20 miles to >30 miles. Lots to think about, but I'm ready for anything, especially when it will benefit so many people.