What is the slowest 5K that would "guarantee" a sub-3 marathon with no marathon-specific training?

Alan Webb’s 5k PR

There are a lot of dummies who can run 16s but race like idiot and can go sub3 (well they are capable but don’t because dumb)

Well, Jakob couldn't finish a half-marathon last year at a decent effort. I'd be surprised if he could run a 2:59 marathon with "absolutely no" marathon-specific training

You would be surprised if a guy who does 20k at 5:40 pace (basically a 75 HM) every weekend for his easy long run could muster a 2:59? This is bait right

The guy is well below 70% MHR running 6:15s at altitude ffs

Most runners doing 50 mpw take a rest day, and the other poster said 6 to 7 days of training.

50 / 6 = 8.33

Did I do that math correctly?

Interesting thread.

So i think the main question is there are very different "5k PRs" and people think of their own reference frame.

Type A: The typical kid who ran track in hs and college, never did more than 90min LR and now wants to check a sub-3 marathon off their bucket list probably needs to run MUCH faster than 19:00. I would guess 16:xx so you are basically running 7:00min miles on your normal runs if you do what most college kids do (run too fast) and then 6:50s in a race feel ok until 30k and you can grind it out. Biggest risk: cramps and stomach probs.

Type 2: The person who never ran track seriously but has a good background in some other endurance sport (biking/nordic ski/trail running) so they are used to going long for hours and used to putting in calories. That could work off no marathon training and 19:00, assuming their 5k PB also came without 5k-specific training in a Park Run or something. Biggest risk: bad pacing.

In between those there are probably some mixed bags where it's not so clear what they would need. But based on intuition and being a statistics nerd, i would bet against any randomly chosen guy over 17:30 if i have no background info.

18.45 or better gets you 3 hours....assuming as part of 5K training you run a regular 90minute to 2 hour easy run as part of your aerobic conditioning..

I ran a 3:05 at a time where I was doing nothing but 7:00/mile pace runs about 5 days/wk for a total of about 50mpw. Just maintenance running for general fitness. About 6 weeks before my local fall marathon I decided to sign up. I probably peaked with an 18 mile long run in that time. The race itself was painful -- rolling hills -- and there was a lot of walk/run-ing happening the last 6 miles. I had to crawl to do stairs for a couple days.

Post-marathon, my running didn't change a lot and that winter I ran a 17:45 5k. I'd never done one before and had no clue how to pace myself. I did middle distance for T&F which for me had ended 5 years before that.

If I could stagger my way to my 3:05 on 50mpw with no specific training background in any event longer than the 800 and my glacial 5k, then I'm pretty sure a 16:00 or faster college 5k runner could handily clear 3 hours on a flat marathon as long as they paced themselves smartly. The last 5ish miles would be very unpleasant, and they'd probably be sore for days, but they could do it.

I don't have a clue about the science involved, but my anecdotal evidence completely supports this. I attempted two marathons when my 5K PR was 16:5x, and both times was running at sub 3 hour pace until mile 22, before bonking hard and finishing in 3:05ish.

On the other hand, a buddy of mine with very similar PRs and training mileage ran his first marathon in 2:55, telling me that he felt stronger as the miles went by, and probably could have been even faster if he hadn't started out so conservatively. So I completely believe that there is a great deal of natural ability involved in hobby jogger marathon running.

Interesting, in my early 40s my 5k->half PBs were very similar: weaker in the low end, bit better on the high end. On 55-70mpw ran a couple of 2:50:xx marathons. But, unlike the OPs question, we ran 20-22+ mile long runs 2-3x per month. A friend who was 20s-1min slower across the board ran a 249, he was maybe 15 lbs lighter than me.

17:32 / 36:12 / 59:58 / 78 high for 5k/10k/10mi/half

Wish i had your 800m-5k speed, but other than lack of 2+ hr long runs, maybe you had some fueling/pacing issues, because at 100mpw you likely had a long run of at least 16 miles, maybe more. Not optimal but still decent. But yeah, agree that sub 1730 5k is no guarantee for sub 3, especially without marathon specific training.

I recall reading (maybe in Running with the Legends by M. Sandrock?) that Rosa Mota never trained >90 mins, but she hammered them hard and ran 2:23 back in 1985. Rob DeCastella once ran with her on a 90 min run and said never again, too fast! She's still running sub 40min 10k at age 65+, maybe helps to be 5'2" & 99lbs.

Some anecdotes in here make me really nervous about my upcoming marathon. I guess part of the fun is finding out if you're any good at it

Oh please.😂

I am a 3:51 runner who sucked at the 5k (14:50 pr). Ran a marathon of one week of training to qualify for boston (I was biking fit though). I ran 2:53, then ran 2:29 in boston. I'd say a sub 16 guy in training should be able to break 3, but if we include young high school runners with low aerobic fitness, I'd say 15:30 for a guarantee

I think it depends a lot on the mileage and the athlete. The 16:00 5k runner who’s clocking 75-80 mpw will probably be a lock for sub 3:00:00, the 16:00 5k runner who’s running 25-30 mpw I’d be less confident in despite probably having more talent.

Of course Jakob and Hocker can bust a sub 3:00:00, they run regular 14-16 milers at sub 6:00 pace. I’d be less confident in athletes like Brazier and Hoey even though I’m sure they can both speed endurance their way to a mid 14:00 5k if not faster.

Most athletes hit the wall at 16 or so miles if they aren’t running adequate mileage/long runs. When I was running 16:00-16:30 in high school at 40ish mpw I don’t think I could’ve run a full marathon at sub 7:30 or even maybe sub 8:00 pace. In college when I was in about 15:00 shape I distinctly remember struggling a lot running 16 miles at about 6:40-6:50 pace because my regular long run was 10-12 miles and 13 or 14 miles had been my longest run at that point.

wild cause i ran 32:40s and was fairly certain I could have broken 2:40

16 mins 5K. Presuming the person is doing no long runs over 12 miles.

I'd guess the most common 5k time for a 3:00 marathon runner is 18 low (eventhough on paper with a perfect build up its more like 18 mid). I know pleanty of 17:XX folks even with some basic traing that didn't break 3. So we have to assume with absolutely no marathon training there will be people in teh high 16s that don't break 3 quite regularly on their first go.

It depends on what you mean by "gaurantee." If its 100% chance its probably closer to 15min. If you are happy with 95% (19 out of 20) then maybe 16:00, but if you just mean extremely likely, say 90% chance or 9 in 10. Then it may be as high as 17:00.

Good post, though given Arop ran a 71 HM last year I think it's safe to say top 800m guys can still break 3 pretty comfortably. Brazier especially has always had good natural endurance and competent in XC even though he hated it. Hoey as a sub 3:30 1500m guy shouldn't have problems with it either.

I think it’s more about training style. My sophomore year of HS, I ran a 16:20. But this was back in the low mileage era so the majority of my weeks were less than 20 miles. There is no way I was running 26 miles at a sustained effort.

Its gong to sound ironic, but had it taken me more training effort to get to 16:20, long runs etc. then I probably could’ve done it.

Tdlr:

16:20 off 20 miles per week = no

16:20 off 40+ miles per week = yes