I don’t have detailed guidance like those above . I’m decently fast as an age grouper and BQ, but I’m not anywhere near your speed.
I find almost no crossover fatigue from swimming to running and only a little from biking as long as keep the biking within reason. In fact I feel like cycling keeps my legs loose during the taper and keeps my taper tantrums in line too. LT and sweet spot intervals are more fatiguing than spinning or short VO2 max intervals. Of course the leg crushing group ride with multiple sprints and hills climbs leaves me shredded for 1-2 days.
My Boston plan this year,
5 and 4 weeks out: peak mileage at 50ish MPW. During marathon season I get to 11-12 hours/week of training, ~ 6 of it running
2 weeks out: Run 37 miles with a VO2 set, hill reps, EZ run and a marathon pace run (12 miles with 5 at MP)
Bike 4 hours: spin, spin, 30min VO2 set, EZ MTB day
1 week out: R: 20 miles with hill reps, short MP set, VO2 set, race prep
bike: 3 hours : EZ road ride, higher cadence set on the trainer
Boston: run fast enough not to freeze !