I'm writing this while recovering from the Boston marathon last Monday.
First, I think most crossfitters would recommend doing a scaled version of the WOD instead of traditional lifting exercises. You will improve much quicker at doing WODs by doing scaled WODs.
The most important run for the marathon is the long run. I don't like the term "long slow distance" because the term slow is too general. Most people recommend doing the long run at a pace that is about 30-60 seconds slower than your marathon pace. If you don't know your marathon pace, use a running calculator to estimate it from a shorter distance. My favorite is on McMillan Running.
Based on 3 miles at 9 min/mi gives a marathon of about 4:30 or 10:30 per mile. That means do long runs at 11-11:30 per mile. This will feel really slow, but you plan on holding that pace for 4:30 to 5 hours so you need to get used to that pace.
Jeff Galloway has a nice site for running on low mileage. He recommends taking regular walking breaks from the very beginning. At 11 min/mile he recommends running for 2:30 and walking for a minute throughout. You might want to experiment with that.
I think you should only plan on doing 1 long run a week or even 2 every 3 weeks instead of 1 every 4 days. This will be especially important once your runs get longer. For me, the hardest part is getting from runs of 20 minutes to 45 minutes. I'd add about 5 minutes a week up to 45 minutes. After that I can add about 10-15 minutes a week to my long run.
If you want to stick to a 4 day cycle I would do a tempo run on day 1 and alternate between an interval run and long run on day 3.
6 months is enough time, but like others have said you will need to get your joints used to the long time spent on your feet. Build up slowly. I find that some long hikes can help.
Also, I think most people have too much fear of low/moderate intensity running. I set PRs in bench press (8x215) and pullups (10 with 45 pounds) while running 45 miles a week. Just be sure to keep up the intensity in your resistance and interval workouts