I hope you are excited as I am with yesterday’s running news!! Check out this thread: New Early Access Feature: Import and Analyze Runs in TrainerRoad!
Training for an Ironman will certainly prepare you to run a marathon. Obviously, if you were not training for an Ironman as well, your training for the marathon would look different. However, it sounds like you want to start making progress towards both goals!
You are correct in saying that you cannot add non-cycling or triathlon Events to your Calendar at this time. Seeing as your two Events are months apart, I suggest that you add the marathon to your calendar as an A Triathlon Event.
The theory behind this is that you will make progress towards both goals (marathon and Ironman) while allowing for a taper before your marathon. If you wish to give more time to running before your marathon, you may wish to choose the Low Volume Full Distance Triathlon Plan, and add in some extra running on top of that (if you feel that is sustainable). If that is not sustainable, the Low Volume Full Distance Triathlon Plan will still prepare you for your marathon!
After the marathon, if you feel that it is appropriate, you may wish to increase to Mid- Volume, and remove the extra running so that you can pay your full attention to preparing for the Ironman!
You can change the volume of individual training phases in the middle of your Training Plan. Check out this article here: How to Change an Individual Training Block’s Volume. It can be a lot of information to navigate in the beginning, @Tominboston , so shoot me a DM if you need help setting up your Training Plan or if you have any other questions for me!