Quick poll for you Iron People.
How much slower is your IM marathon PR from your regular marathon on its lonesome PR?
Ooooh, good question, I’ve actually been wondering about this myself. I’m hoping about 1 min/mile slower, so that if I can cruise through a sub 2:55 marathon, I’ll be shooting for 3:20-3:25 IM marathon, is that reasonable?
My guess is that the top pros lose no more than about 10% to 15% on a good day. I would also think that a well trained AGer would be in the neighborhood of 15 to 25% loss. Any other experiences out there? I can’t personally speak to it as my only marathon is in an IM (and I had some ITB friction issues so it would not be a good example anyway), but looking at various results over the last two years or so it is all over the map and there are tons of variables. If you are an inefficient swimmer and have to work harder to get through the water then that will affect your run, especially in the long distance stuff. Same goes for the bike. Mess up on nutrition, temp climbs to 100, you decide to hammer a hill on the bike or just have a bad day, etc. and your sub 3 turns into a 4+ real fast.
Bump 
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Marathon PR: 3:58. IMNZ run split: 4:46.
Better/more bike training would have helped my run. A lot.
Joe Friel in Triathlete Training Bible implies that for a well conditioned athlete (assumed on my part) that one should be able to run your stand alone marathon time plus 20% for an IM marathon. I have no personal experience to date. Ask me in September.