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As for speed, this is just my opinion, but if you can do some HIIT and sprint work, it will will really get your mile pace down
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While tempo work, hill repeats, interval training, fartleks and many other types of faster and higher intensity work can be very helpful, I don't think actual sprint work has much use in marathon training. From my personal experience, all out sprint training is a direct path to injury when you're putting in a lot of miles.
FIRST training program
I think this program would be very good given the time you want to allot to this. The long runs are very very important. You're going to have to go well beyond 15km on a regular basis in your training if you want to have any shot at 4 hrs.