As in my response to the OP, I'm inclined to think that you can have your cake and eat it too. For most of the block, double the running in your normal triathlon training and cut the bike/swim just enough to accommodate. Don't let swim or bike workouts destroy your energy levels, but you don't have to keep it all easy either if you're feeling good -- if you are running sometimes on bike-tired legs, that's not bad training for a marathon. If you are on the verge of a run injury, cut back running for a week and quiet your mind with some hard work on the bike or in the pool (for me this is really valuable psychologically, if nothing else). Make your training increasingly run-specific in the 6 weeks leading up to the marathon. Do #ST100in100 -- it's exactly the spirit of run training you want to make marathon training compatible with triathlon -- focus on consistency and frequency, but still leaves space for the other sports.

Consider a hypothetical triathlon halfway between a 70.3 and a standalone marathon (say, swim 1k/bike 45k/run 30k). I think if you train for that hypothetical race well, then you'll actually be in damn good shape to race a standalone marathon. And you're in really good position afterwards to shift gears back into 70.3 training without having lost too much ground on the swim and bike.