Two words:

Bike fitness

Edit: in all seriousness, I don't think you can rule out their bike or swim fitness. If they are not running to their ability or performing to their ability, then it's really all about bike fitness. If you aren't fit enough on the bike, you just can't perform on the run. Even if you are a 2:30-2:40 marathoner.

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I agree with Steve. I've met and/or read about BQ'ers that go 4:30+ on the IM marathon. I "think" a lot of them think since they're runners that they'll be just fine on the run and completely underestimate what that 112 miles will do to them if they don't have the bike fitness and execution more or less dialed in.

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I'm actually going to go the opposite direction on this. If I get to work with an athlete that has either swimming experience or running experience as a youth, its almost like a cheat code I feel.

For the triathlete you are explaining, I would guess that they are lacking focus or structure with swim/bike workouts. They know how to train the run because they have done so from a young age... most pure runners I find can become very fast triathletes if they learn how to focus correctly for swim or bike workouts.

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COROS Sports Science

I'd agree on bike fitness..

In addition, a good runner wants to be able to run well at the end.. it takes some time to figure out how hard you can ride and still run decently. With experience they will improve that balance.

The same could be said of good cyclists that aren't good runners.. often are missing run fitness.

There was an article I read once about someone trying to get pure Kenyan (or Ethiopian - I don't remember) runners into triathlon and they were god awful. Just because you're good at one thing, doesn't mean you'll be half as good at another.

Triathlon is not 3 sports... its 1. Once you realize that, it's a lot easier.

Yes, I've read about the possibilities of Kenyan runners abilities with triathlon and I agree, the top Kenyan runners are so light, they might not have the leg strength for cycling. It's a interesting phenomenon though. So maybe the best runners aren't that "strong" per se? As in they might have fast-twitch muscles (even the marathoners)?

This is slowtwitch, so if you don't have science, you have nothing.

But yes, that has long been my feeling. All the "fast" runners I know are not strong, which does not play well in anything beyond an Olympic distance triathlon. They might be able to have a fast 70.3 on a flat course but until they build strength (slowtwitch muscles), they won't be able to transfer speed.

And personally, everyone I know that is fast will never allow themselves to slow down enough to build strength.... typically because of ego.

Correct, the kenyan experiment didn't work out like they thought it might. I think this is an extreme representation but I could also make the argument that some pro cyclists have turned into very good triathletes (different discipline than OP though). All I'm saying is if you get a strong 15 min 5k'er, they have a high potential of turning into a strong triathlete with a year or two of development.

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COROS Sports Science

I would actually expect most sub-3 marathon runners (or close to that) to be close to FOP bikers from HIM down to sprint distance just on their run ability alone.

I have yet to meet a 3hrish marathon runner who was a joe-average MOP-AGer cyclist. They're all pretty good or will get good fast after getting past their fears of bike handling.

I do think there's a difference at the IM level though - at that distance, it's so far/long that you're really exposing the runner's weakness on the bike, and they'll either eat it late in the bike or (especially) the run where they go from 3hr marathon to 4+.

The real killer for runners really is swimming. ESPECIALLY at sprint/oly level, where it's closer to parity for the SBR distances.

I fell into this category - was a pure marathon runner (albeit not sub-3, but not too far off 3), went to tri, and found that I could put down top 25% AG bike splits in Olys with very little training. But the swim?? Wow, that killed my results. I'd finish top 8% run, top 20-25% bike, and then bottom 10% swim. That was my whole 1st year of triathlon. And that swim alone dragged me down to the 50% AG mark in triathlon.

I'd say it's the SWIM that kills the runners for up to IM. At the IM distance they can survive the swim without as much damage even with a near-2hr swim, but their bike weakness gets exposed either on the bike itself, or by blowing up on the run which will cost them 1+hrs.

There is another problem with good marathoners attempting a fast IM. They tend to be on the shorter and/or slimmer side of the build. That works well in marathons where weight is a big limiter. However, in IM the top men tend to be around 160-165 lbs/6'1"-6'2" type of a frame. The smaller guys trying to go hard on a 5 hour bike ride get beaten up too much - and that takes their legs away for the IM marathon. Bigger guys (ie all the top germans in Kona) hold up a lot better and save their body for the run.

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Next races on the schedule: none at the moment

Running fitness does not equal cycling fitness, entirely different.

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I see your point here on the size. Taller, but slim, seems to make for the most watts/kg in cycling if the strength is there. There's no doubt that same 6'1"/6'2" guy isn't going to be in the Olympic marathon. I did hear about a study not long ago where they measured the limbs of top level Kenyan runners and they found the fastest people to be between about 5'4" - 5'7" and these people also had very small lower leg bones and calves. It was a really interesting analysis...I wish i could remember all the stats.

I'm a decent runner. (1:40 13.1 PR, 3:33 26.2 PR)

Not great, but decent. Behind the fast guys, but faster than the MOP.

I've done two 70.3s and one full IM. All three, I was perfectly middle of the pack.

I'm a crappy swimmer though. Like last 25% of the field in every Tri I've done. And I'm mediocre at best on the bike. My 56 mile PR is like 2:54.

My run fitness absolutely did not translate to the bike. At all. Which is why I've been focusing on it the most for the past 8-9 weeks. Hopefully I can break the elusive 20 MPH barrier on the bike in my two sprints in May.

This. THANK YOU!

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I would still say there is a big overlap with run/bike for non-FFOP AGers.

I seriously doubt any sub 3hr marathon runner will be a BOP or even dead MOP cyclist for more than 1-3 months after getting comfortable on the bike. There's no way any sub-3 marathon runner is stuck in the BOP on the bike, unless they're racing on a mountain bike.

It's very different from run/swim where there seems to truly be little overlap, where that 3hr runner will likely be stuck in the BOP for quite awhile until they really commit to serious swim training.

Alan Webb did better than most on this board will ever do in both sports.
Carol Montgomery did quite well in both sports.
You have to develop the cycling or your legs will just be mush on the run no matter how good of a runner you are. Even if you're not a fast cyclist, you can build the fitness so it doesn't destroy your run legs. Not being a fast Uberbiker doesn't mean you're not fit on the bike. I suppose you could say fast is a relative term though.

Triathlon is not 4 sports (adding in transitions here) it's 1. (pink)

I come from a college run background and after 1 year on the bike my bike splits place in the overall field where my run splits do. You can't expect to bike just because you run, but having a solid run background is a great place to start biking from. If a runner puts in the time on the bike, 9/10 are going to do very well. Better than a no background person.

Benjamin Pardes - WC runner turned duathlete back to WC runner, got 3rd at one of the big marathons after his du stint.

When it comes to pure runners I'll go out on a limb and say yes bike fitness but unless they've spent a lot of time in the pool, the swim will ding them as much as the bike will.

never underestimate how being a shitty swimmer or swimming shitty will kill the rest of your IM

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Everytime this guy comment I always take out my notepad and jot down he says.
Hes already turned my running around and he doesnt even coach me.

Note to self : more swimming.

Well I'm a 2:47 marathoner and a 23-25 min HIM swimmer, but it's taken years of heavily bike focused training to get my bike anywhere near the same ballpark as my run and swim.

One factor I didn't see mentioned in my quick skim of this thread, at least explicitly: running economy. That is, if somebody is particularly economical, they can be a pretty decent runner despite not having exceptional aerobic fitness.

On the flip side, there are always the big guys with perfectly average aerobic fitness but an exaggerated sense of their personal abilities just because they can power a bike along the flats at a reasonable clip. Then the road tilts upwards, they get dropped by women and children, and they are left scratching their head why...

I think I fit into this category, but I don't really know what the middle of the pack really means.
I'm a better runner than triathlete, I just find that in the triathlons I do, there are more good athletes than in the small running races I do.

As a percentage of the world record I'd be a better triathlete, but I definitely feel more MOP in triathlon than in running.

"They are good runners, but even despite committing hard to swim and cycle training, their ability doesn't cross into SBR."

I'm guessing if you got a look at what their "hard commitment to swim and cycle training" was, most of the time you'd see that they are not training that hard even if they are training a lot of hours. Lots of easy running works, lots of easy cycling and swimming doesn't.