That all depends on the bike. If you are well prepared and don't over do it on the bike, you will probably run about 1 minute per mile slower than stand alone marathon pace. Over do it on the bike and you will be walking the second half of the marathon.

Post some times of your PR 13.1, 26.2, 5k, etc?? Might help predict ....

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Colorado Triathlon Company, CO2UT 2021, Crooked Gravel 2022, Steamboat Gravel 2022

5k - 17:04 (Haven't run this competitively in a few years)
10k - 35:55 (Haven't really run this in awhile either)
Oly 10k - 37:50
13.1mi - 1:21:30
HIM 13.1mi. - 1:24:27
26.2mi. - 2:45:05 (1st half 13.1mi - 1:21:15)

A well-executed Ironman will have your run split within 10% of your marathon time.

Lots of people people fail to execute their first iron distance well.

Shouldn't having a coach help mitigate pacing issues though?

x2 on both of those.

Yes, a good coach would definitely help you work out a good execution plan. And if you do, like I said I'd expect you to be within 10% of your marathon time.

Plus or minus of course.

Fixed it for you.

You would do exceedingly well to run within 10% of your open IM time (i.e. < 3:02 IM), not impossible but difficult. More realistically based on your times I'd aim for an IM marathon around 3:05 - 3:10.

Key to achieving this will be 1) ensuring you don't over cook the bike, 2) are well fuelled/hydrated coming off the bike and 3) don't start the marathon too fast.

FWIW my run times are very similar to yours and I generally execute Oly 10K & HIM pretty well. In my 2 IM's I've underperformed on the run (3:21 & 3:22) due to all 3 of the reasons above. I didn't seem to learn any lessons the 1st time :).

One other thing I'd add is that IMO bike strength is key for an IM run. I don't cycle much so I feel that even if I pace the bike well I'll still struggle a bit on the IM run due to the 180K taking a fair bit out of me. So for an IM as well as pacing the bike correctly, you also need to have a reasonable number of cycle k's in your legs to execute the run well.

coaches are less needed for this type of knowledge with the amount of data on the internet - unless you are quite lazy or like to find a scapegoat for failures. what a coach is good for is critique of form.

This may help....VDOT Chart for HIM & IM races.

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