Ultra run training effect on Ironman

well I will comment even though I can't answer your question because I think your question is important and deserves much discussion.

The answer will depend on how you are running those 72 miles and how much you usually ran, I think. I can't see your logs but if you following a barryP method with maybe some 2 a days on road racing type surfaces and terrain and are running more than you used to then I imagine your stand alone run performance would be better. But, if you are not riding then your loss in bike fitness may reduce any gains in the marathon of an IM.

If your training plan involves running up and down mountains for 4 to 5 hours at a time then I think your stand alone marathon would suffer as would your IM marathon....unless your marathon was Pikes Peak.

Of course this is assuming that you are at some sort of peak physical condition prior to your initial IM and ultra training. If your ultra training actually makes you fitter then you have the added variable of how general fitness would effect your marathon or IM marathon time.

I think the key would be to not try to lose too much bike fitness while you train for your ultra. I would think that if you could retain a good portion of your bike performance while increasing your run volume then when you return to IM training you will start at a better starting point than you did for your pre ultra IM.

But you probably know all this and were just looking for a real live person who has been through what you are doing.

I hope this makes sense.