Here is one of the most popular producer's of marathon training programs.
Here is Advanced 1: The training programs for Advanced 1 marathoners follow a progressive buildup–similar to that for novice and Intermediate runners, except you start at 10 miles and peak with three 20-milers. There is also more training at marathon pace (usually Saturdays, the day before...
www.halhigdon.com
If you drop to down to "advanced" program for someone who is already in good running shape, note:
Cross-Training: There’s no cross-training scheduled for advanced runners. Sorry, but we don’t have a place for it. If you feel you need to (or like to) cross train as a means of avoiding injuries (or as a variation from running), you can substitute an aerobic workout (swimming, cycling, walking) for the running you might do on Mondays or Wednesdays.
Also note that rucking is not an aerobic workout, it is Muscular Endurance, so it wouldn't even apply to the exceptions above if an advanced runner wanted to implement it. Again, if you want to put up a good time in a marathon then, even as an advanced runner, it is going to take a considerable amount of training volume over several months to up your race pace to a competitive time over 25 miles. You simply don't have the training capacity for it. If you want to David Goggins it and do some suffering for a sport unto itself, then go for it, but as to the original question: will "rucking help with training for a marathon", the answer is no, its going to impede the training block. If you want to ruck, then ruck. If you need to ruck for a hunt, then ruck, but if you want to focus on the marathon, then run.