Well, back in '02 when I ran alot I followed Eric Harr's training advice:
Weight training, 30 min run.
60 min run
60 min run
Weight training, 30 min run
2 hour run
The running was always a LSD pace (70% hr). You ran for time, and let the distance be whatever you got.
As you can see, I wasted alot of time running (good old hindsight). I did about 5 months (never running farther than 3 miles in my life previously) of training which ended with a marathon. I made sure to get two 3 hour runs to get accomodated to the pounding. I pulled off a 4 hour marathon weighing in at 205 pounds.
Now I blindly (ie I've not tried it) recommend the following to get you ready for a marathon:
Four to Five months prior do Crossfit routines, but one day a week do your LSD as your WOD. First month, each run is 60 mins for distance. Second month, make one week a 2 hour run for distance, the other three weeks are 1 hour for distance. Third month, two 1 hour and two 2 hour runs. The last month you can do 1 hour, 2 hour, 1 hour, then 3 hour run. I think that your body should be conditioned to handle upto a 5 hour running endeavor.
Granted, as your metabolism will be spiked thanks to CrossFit, you should have a sub 4 hour time. Just USE A HEART RATE METER to pace yourself. Many may decry a HRM, but I think it's the best tool for endurance events for hack runners like myself.
Also, use a Camel Pack with either Gatoraide or appropriate drink to replinish your sodium balance. Drink about 1 liter per hour. That's for ALL your LSD running.
The big thing I've noticed after doing CrossFit is that running any more than once a week does not seem necessary. You're not getting too much benefit from it, you're just getting your bones used to the constant impact.