Yes, i did the (first) full marathon 6 weeks after the half ironman one.
I switched the training program to one designed for the marathon (the one from Furman institute – FIRST Marathon).
It went so-so, but i think that the biggest impact was given by the fact that during the week of the marathon we have organized a conference, so there was not enough sleep / rest 🙂
I did much better at the 35km long run session, 2 or 3 weeks before (lower hr, no cramps). I have to give it a new try this year (i am currently targeting also the october one here in Bucharest).
Can i share some specific experiences / answers?