J,
Congratulations on your upcoming first Ironman! Can’t wait to hear how you do.
For sure I’d load the 70.3 plan first, and then load the IM plan for the remainder of the IM plan.
There are 12 weeks between your 70.3 and the IM. You should take a week off after the 70.3, leaving you 11 weeks to train for the IM. This puts you starting at Week 13 of the IM plan, which has a total of 11.5 hours, and a long run of 1:50 and a long bike of 3:30, and a long swim of 3,000. Your longest week of the 70.3 plan will have 11 hours total with a long swim, bike and run of 3,000, 2:30 and 2:00, respectively.
So, your first week of the IM plan will feel just like the last week of the 70.3 plan. For you, the timing is perfect.
I’d recommend the Run Pace and Bike HR plan over the Run HR plan. Pace is going to be more reliable for you overall.
David