Marathon time vs IM run split?

Do a search over on ST.  IIRC, consensus seemed to be that about :30 over open marathon time is a pretty decent IM run.  But the gap is narrower for the pointy end of the field and probably wider in the BOP. 

As to what you can expect, it all depends how well you pace yourself.

Mike,

It really depends on how you train and how you approach your race. There are those that tear it up on the bike and then walk most of the marathon, and those that take it easy on the bike and run the entire marathon and finish at the same time. For example two athletes that I know very well did the same IM race this year and finished within about 30 mins of each other, one was a 5:30 bike/6:30ish marathon , the other was a 7:00 bike/4:30ish marathon.

For your first IM I wouldn't get to caught up in time goals, your only goal is to finish the race in less than 17hrs, anything else is gravy.

To answer your original question, there isn't any rule of thumb for figuring out a time or splits (yes I know about all these IM calculators out there). There are people who are 3:30 marathoners that take 6hours to do the IM marathon, there are those who run 4:30 marathons that run the IM marathon in 4 hrs flat. It really just depends on your training and how your body adapts. 

I've run one marathon (December 05) and one Ironman (IMFL November 06).  So, I'm no expert.  But, here's what I think.  I ran my first marathon in 4:33.  I ran the IMFL marathon in 4:08.  I attribute the improvement to all the training leading up to IMFL and my race plan.  Because of my lack of marathon running I don't really know what my open marathon time is.  I'll find out on December 10th when I run the White Rock marathon. 

I think if you train right and race a smart race you can run a marathon very close to your open marathon time.  By race smart I mean take it easy (z1-2 and maybe z3 the last 20 miles) on the bike.  Have your nutrition worked out and working for you on bike.  Then on the run the same thing.  Take it easy the first 18 miles of the run and then let it rip the final 8 miles.  The final 8 miles of the IMFL marathon I was passing people left and right (it did hurt). 

TJ

If you pace well, your IM marathon split should be about 15-30 minutes slower than your standalone marathon.

I define 'pacing well' as being able to pull off an even split to a slightly positive split marathon run.

For example, I ran a 4:09'ish at Chicago in 2005, and ran 4:33 in my first IM in 2006.  ( Though that IM split includes 4 minutes in a porta potty, and 4 minutes at special needs changing socks and trying to tape up a toe ).

I'm curious.  Have you thought about going easier on the bike so that you have a better run?  Is a 5:30 bike a z1-2 ride or is it a z3 or z4 ride?  Let's say you did a 6:00 bike and this enabled you to run a 4:00 marathon, then you would be :15 ahead of your IMC time.

TJ