Ryan, thanks for posting on the forums.
I would not have concern about the transition from Marathon to 70.3 training and can’t think of any significant adjustment you’d need to make. The time between the marathon and 70.3 is so much that it’s really two separate seasons. There is no disadvantage, only the advantage of entering into your 70.3 training with exceptionally high run fitness.
Remember, with a zone-based training system (like we use with our 80/20 plans) you can throttle the difficulty of the swim, bike, and run workouts by performing them at the bottom or top of the zone. In your case, you can perform the run workouts at the very top of the zone to increase the level of difficulty, but keep the bike in the low to middle as your fitness there wont’ be as strong as the run at the beginning of the 70.3 plan.
David