Perfectly doable, you just need to set your goals appropriately. Since the marathon is more important then specifically train for the marathon, don’t train for the HalfIM, but keep some swimming and cycling as cross training (maybe have a look at the FIRST plans?). Turn up and do the HalfIM for fun, but don’t worry about the result, take the bike super easy and try to run at your marathon pace, if feeling good then up the run pace towards the end.
Most important of all, have fun !