Bike ability vs Running ability

You could also look at it this way...if you're comparing your open 5k and 10k times to elite runners...then yes, your times are somewhat MOP.

But if you're finishing top 20% on the bike at Oly's and Sprints, I'd bet if you showed up to a bike race such as a TT or road race where the real elite bikers show up...you'd probably be MOP as well. 

My son is 15.  Nearly 27 mph on 10 mile bike TT, and 16:23 stand alone 5K, and sub 5 minute 500 SCY swim. He works is arse off on all three.....that's your answer. 

You can call it any way you like....the really fast guys work harder and, unfortunately for the rest of us , probably had a genetic edge to begin with.

If you bike more you will get faster.  In that regard it is the same as running.

Also, can you swim?  If so, find a draft legal race......you can hide in a pack on the bike so it's not nearly as important.

For me, I find that I'm relatively strong on the bike and mediocre at best on the run.  In looking at my body, I can see why.  I have above average strength in my quads and calves and below average strength in my hammies and glutes.  I can produce a decent power/weight ratio on the bike and climb better than most.  I had a run analysis perfoemed and the instructor said I was the most calf dominant runner he's ever seen.  Most good runners get the majority of their power from their glutes.  I don't get much from there.  I work on it, but its slow to change body types and run forms.

Anyway, look at your body.  A bit lacking in glute strength relative to other leg muscles?  Probably won't be a great runner.

You need to run a lot more...consistently...over a long period of time.  Once you get up to that fitness level, it doesn't take as much to maintain it...but most sub 18 min 5kers likely have spent years running nearly everyday, around 50+ mpw.  Of course there are outliers who can do it on much less...and there are also outliers who need to do much more in order to get to that level.

I've been on and off bicycles since I was a kid, but only started running a couple years ago.  Yet I'll run 10K at a training pace that's faster than your PR and carry it for another 11.1K.  But you'd probably crush me on a bike, especially on a flat course.

Crush being a kind description of how that would go down.

Then again, I'm short, thin, have skinny legs, and I'm built like a distance runner.  It just took me 38 years to figure out that I should be a runner.

My AG in the one sprint Tri I've done was won by a cyclist though.  His time in the water was mediocre, and his run split was mid-pack, but he was 6 minutes faster on the bike than the 2nd place guy (who ran a 19m 5K). 

I don't think that's true.....or maybe it's just not true for young people.  I spend a considerable amount of time around running teenagers.....and many of them are sub sub 16 - 17:00 5K runners.  Most run ALOT less than 50 mpw....my son runs less than 20.  He swims a ton, and rides quite a bit, but we keep the run miles very low on his growing body.  It is the same for his peers who are also very fast. 

I think that most sub 18 and certainly sub 16 minute 5k'ers are born.  Yes, you have to put in the work at some point....but as you know, running 50 mpw won't make the majority of people sub 18:00 for 5K.  They'll get faster, but most people can't run a sub 18:00 5K no matter what they do.

If you're going to cite young athletes who are competing at the top of the state and national levels, then you are talking about some of the outliers...as you mention that some of them are "born."  Especially those who can go sub 16...which is a HUGE difference from sub 18. 

Jason...my point, though admittedly not well put together in my post, is that most of the folks who can run sub 18:00 didn't get there running 200 miles per month....and most people who run 200 miles per month will never get there.  In order to run sub 18, and yes, sub 16 (go spend some time at your local High school XC invitationals.....you're going to be stunned at how many of these kids run 15:XX - 16:XX for 5k and aren't at the top of anything) you have to have ability that in itself is an outlier...because 99% of the population can't get there, and I don't care how much they run.  Most of the 1% who can get there run a lot less than most people think they do.  I disagree that "most of them spent years running 50+ mpw"....ESPECIALLY triathletes who can do it.