Running a "downhill" marathon affects your body differently than a flat or rolling course would therefore you should train differently.
I've asked a couple of my super fast friends to share some advice for training for a downhill marathon and I also found a great article from the September 2009 issue of Running Times
"Owner's Manual: Avoid Downhill Disasters" By Brian Metzler
Here's what they had to say:
"Get your quads strong and then train on the course if you can. Put in at least one 10 mile tempo run each week at your goal race pace." Dave Mora (More advice from Dave)
"Train specifically for your race. Winthrop contains a lot of long gradual downhills.
So train on long gradual downhills. Do some of your speedwork on long, gradual downhills. If you , practice concentric and eccentric contractions with your quads. And if you are carrying around some extra weight-- lose it, within reason-- a lighter package pounds less and puts less stress on your joints." Susannah Beck
"It's certainly possible to run down a new PR on this type of course, but only if you've trained properly." Brian Meltzer (see his Running Times Article for tons of great training advice)