See below. There is a thread about this somewhere too.

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https://www.strava.com/athletes/4391866

Whoa, that's interesting. Where does that come from? (looks like it's from a study of some sort) I did a couple of searches, but nothing obvious came up in the results (based on the thread title).

No idea where that comes from, but based on my own results, it feels fairly reasonable. My datapoint from last year was a 1:24 HM and a 1:30 run in 70.3. Both races within a month of each other.

That seems about right to me :)

OF course the longer you spend in the two sports prior, the more off the run % will be. But take your sprint and olympic distance, for ITU it might be that 5% on a bad day, usually much better. But for non draft a good run would be closer to 7 or 8 %. ITU allows drafting on the bike, so even though the bikes are hard, there is a lot of resting of the legs, as compared to a steady, anaerobic threshold type ride..

OF course all of this is dependent on how one paces, you of course can pace a race and then run your PR, using the first two as a moderate warm up.

I'm not sure where it originated from. I think it's pretty interesting and should give you a ballpark of what you are capable of, provided your training, prep, recovery, pacing etc. is on point.

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https://www.strava.com/athletes/4391866

I've been using that chart as well, saved a link to a thread containing it, think it is pretty old (2012 or older). It is fairly good, again as you said for a fit athlete, however the only thing that I caution about it is taking into account heat factor. It is rare that we triathletes get a race like Maine 70.3 where you can run in 70-ish degree weather (after swimming in sub-60 temps!), many of our races, especially longer distances, are 80+ where I find even the slower estimates on that chart are difficult to attain. And I say that because it is highly likely that stand-alone PRs in running are done in even cooler conditions in the spring & fall. Likely but not always.

My n=1 is a smashing PR at the Eugene half marathon where it was just beautiful, 60 degrees, light winds, somewhat cloudy enough to shield the sun. A month later, doing the Hawai'i 70.3 (extreme comparison, I know) I also had a great run, 18 minutes slower than Eugene.

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808 > NYC > PDX > YVR
2024 Races: Taupo

I have found the chart above pretty accurate for me in general, though it will vary for everyone. Specifically for 70.3's, I am typically 8% slower (1:39 HM, 1:47 HIM) but running is my weakness. Stronger runners will likely close that gap more.

Wattie Ink had a blog post that summed up simply as:

• Sprint tri vs. 5k: 4-7%
• Oly tri vs. 10k: 4-7%
• Half Iron run vs. Half marathon: 4-7%
• Ironman run vs. Marathon: 9-13%

Here's another comparison I had found on slowtwitch at some point. it aligns well with the chart:

• Tri 5k approximates (or vice versa) 10k open run pace
• Tri 10k approximates 13.1 open run pace
• Tri 13.1 approximates 26.2 open pace
• Tri 26.2 approximates everyday run pace

I've always thought this was a pretty good representation as it gives you a fair amount if wiggle room on both sides (faster/slower) as opposed to the dialed in chart numbers above. Your triathlon run split should approximate the next distance up (double) in open running.