My views may be biased by personal experience, but my best marathon came after a season in which I regularly rowed with a 4. We weren't great and my technique is particularly poor, but I found that there was significant complementarity between the sports.
You likely should identify the sport upon which you are focusing - rowing from the tone of your question - and view the other as cross-training. The only significant difference from normal training for rowing is that you won't stop your running at 5 miles all the time. At your peak, you will likely do something in the 10 mile range and up to a 20-miler on the weekend. However, if all you want to do is to complete the marathon, don't plan too many 20s. Three should be enough with a couple of weeks in between.
Of course, if you plan to run through the winter, you can do an early spring marathon and arrive at the start of the rowing season in superb shape, which might be an even better plan except for the fact that you could find yourself enjoying the running sufficiently that you decide one in the fall makes sense too...
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