Marathon 6 weeks after doing Ironman?

I'm not going to tell you if you should do it or not, you have to be comfortable with your own training to do so.

However, with the IM 6 weeks before NY, you won't be recovered in time to do a 20 mile training run and you won't have to either. Your fitness will be up you just need to maintain it with some easy runs.

This is between you, your body and your training.  I am almost three weeks recovered from IMWI and I don't think I could manage a 10mile run (I could ride a century or swim a few miles, but not run).  This is how my body reacted (in large part because I am a horrible runner), but I had a friend who raced a 70.3 (just missed a championship slot) and three weeks later qualifed for Boston so to each there own.

 Good luck in whatever you decide.

For the last two years I've done a hilly 15.5 mile race 4 weeks after IMCDA.  At 6 week post IM I have done a 2.5 hour (18ish mile) training run.  But I have not felt fully recovered from IM until 8 to 10 weeks out.  I know that this will be your first IM, will it be your first stand alone marathon?  If NYC is going to be your first marathon then I would have to say that you are setting yourself up for a bad virgin run.  Would I do a marathon 6 weeks after IM, heck no.