This was posted a while back by Paul Petersen, I think it's very sound advice.
"If the most common question I get asked is, 'How do I run a fast marathon?,' the 2nd, 3rd, 4th, 5th ... most common questions have something to do with diet, running form, sleep, footwear, stretching, weights; anything but actual running. American runners seem to have an unending fascination with all these extra-curricular activities, yet we continue to get slower at running marathons ... If you want to improve your performance in the marathon, stop worrying about minimalist shoes, caveman diets, and new-age running form, and start worrying about getting out the door and running a little more than you did last week. High mileage works!"
- American marathoner Pete Gilmore telling the truth.
I can only speak from my own perspective. I did not, and could not qualify for Boston until I was consistently running 60-70 miles a week for a year, running 6 days a week. I dabbled in triathlon training a few years back. I love to cycle and miss it dearly. I had to cut 12 minutes off of my marathon time to qualify. I couldn't do it for three years. I kept trying different marathon plans and the best I could do was to continue to come in at 3:42. One thing that remained the same over this three year period was my annual mileage, which averaged 35 miles a week.
Once I doubled my mileage, it was a whole new world for me. I improved my race times at all distances. No magical speedwork, lactate threshold this that and the other, just mileage. Every day running, doubles three times a week and usually one day off. My bike sat in the basement collecting dust. It was very hard, but doing both was impossible for me if I wanted to be a Boston Marathoner, husband, father and full time worker.
You need to cut 25 minutes off of your marathon. It's time to decide what is most important. Keep in mind, hitting your BQ time might not be enough to get in. Registration last year was a competition. If you want to actually run the marathon you probably need to shoot for an early registration benchmark which would begin at a sub 3:04:59.
Good luck, and be careful. Aggressive goals take sacrifice for you and your family. Keep it in perspective and keep it fun!