The heart can handle it but the legs can't

You need to run more.  If you limit yourself to 3x/wk, you will be limited in the pace of your progress.  Not saying you can't make progress (you can), just that it will take time.  For now, start your runs slower.  You don't want to be fading at the end.

X2

With a 5k of 21min and a marathon of 4hrs, the answer really is that easy.

Run more. A lot more, if possible. If you're doing it right, almost all your miles with be at conversational pace but you will be running so many miles that your legs feel kind of dead by the end of the week. Build up slowly, of course.

The marathon, as you're finding is a race that places a heavy premium on overall volume, and less so on raw leg speed for AGers. You will essentially run the whole race at a high-aerobic zone, with the exception of the last 10-15 minutes if you have any gas in the tank, so doing high-HR workouts will have much less of an impact on the marathon result, say, compared to a 5k.

And for what it's worth, if you're using the Mcmillan calculator or other calculators to estimate your marathon potential of your 5k time, in most cases it takes people 70 miles per week of training to hit that Mcmilan estimate, and that's on a very favorable (flat/downhill) course in ideal conditions. So adjust your expectations accordingly.

I'd recommend trying out a marathon structured plan with mileage targets just slightly higher than what you're doing now to get on the right track if you're serious about it.

Yaz.....where is the source for that?  Not ready to call BS,  but that seems crazy to me.....280 miles per month?

Your times and goals are similar to mine and have caused me great frustration.  Last 5k 21:12 with a sub 4 -hour marathon goal in a few months.  Last marathon in October 4:09.  Last year, I ran 4 marathons and they all imploded in the last 6 miles despite averaging 100 miles a month for last two years.  Bottomline is that 20-25 miles a week in nowhere near what it takes to really drop your marathon time.  I thought that all the biking and swimming I was doing should make up for it.  From a cardiovascular standpoint, the crosstraining certainly helped.  I never felt gassed.  Just did not have the leg strength that comes from running lots of miles to carry me through the final miles.

I'm up just over 40 miles a week now (working towards 50) with some limited speedwork and it has made all the difference in the world with my running strength.  I think it is really tough to get in the kind of miles you need without running at least 5 days a week and preferably 6 or 7.  The additional days do not need to be fast or long for you to realize the physical adaptations that you are looking for.

No....it depends heavily on how fast you are at shorter distances.  There is no way in hell a person who runs a 30 minute 5K has to run 280 miles per month in order to run a 4:13 marathon.....that's just wrong.  By the same token, my son runs a 4:30 mile and it has him predicted at a 1:57:00 marathon......NO amount of running is going to make that happen. 

On the other hand, when I plug his 5K time in it shows he can run a 2:13:00 marathon.....yeah, I can easily see the 280 mile per month number being relative, probable even light.  Then again, most calculators can't agree either.....another has his marathon time at 2:34.

Like all things with running, blanket statements don't usually cut it.

This will be interesting discussion this weekend here......we are hosting a couple of sub 2:30 marathoners.....brain picking time.

whenever people ask questions like this on BT it is always extremely helpful to have up to date training logs.  there is no way to know if you are running enough or if your ratio of hard/easy runs is good.

off topic but in response to your post: if your an adult onset swimmer then you know you are only going to improve so much there before you are going to hit plateau after plateau... spend most of your training time elsewhere. an extra 3 hours a week running/cycling will pay off a lot more than 3 more hours swimming when it comes to general fitness and overall race time.

I'll take your word for it.....and yeah, I know that serious marathoners are always around that 300 mile per month mark at peak training.

Here's the calculator that had the crazy numbers.....maybe the person who did the calculations was on Meth.  

http://easycalculation.com/sports/mcmillan-running-calculator.php 

Wrong site. I don't know where you got that site, but that one seems way too aggressive based upon what I'm entering in.

This is the REAL Mcmillan calc website, with the calculator we're all referring to. It's very common knowledge in running circles and as mentioned above, one of the most common questions on any marathon forum is "Can I hit my Mcmillan estimate?" which is why that 70mpw number comes up frequently when talking about hitting the estimate.

http://www.mcmillanrunning.com/

I've played around with Mcmillan (and I just clicked on the first link to find that screwy calculator)......I still maintain your 70 mpw number gets more accurate the faster you are.  Using the 30:00 5K example again.....no way does someone need to run 70 mpw to hit a 4:52 marathon......but moving down to the 5K times I'm watching, sure, 280 miles per month to hit a 2:34 marathon seems about right based on how little they run to hit 15:XX 5K's.  Like I said, later tonight I'll have some sub 2:30 marathoners at the house so maybe sometime this weekend I can eavesdrop when my kid is grilling them.

They will likely have to. The odds that someone running a 9:50/mi pace 5k all out, will be able to run faster than a 4:52 marathon on low mileage are almost zero.

I think you're also neglecting the improvement factor as well though. It's def true that a 30min 5k runner whos let's say, M45, will no longer be a 30min 5k runner if they ramp up to 70mpw like on Pfitz. In fact, there's a good chance they'll becomes a 22 min 5k runner or faster. You would have to then update your times in the calculator.

But for sure, there are a lot of senior runners (55+ age group), many of whom are Boston Qualifiers at around the 4-5hr mark, who in fact run all of those 70 miles per week to get their Pfitz times. Even these runners say the same thing - you need on average 70 mpw to hit that Pfitz time. (Some require less, more actually require more.)

I think the differences in people must be huge. 

Last night I passed beers to a couple of guys who stayed at our house while they made their way home from a coaching clinic.  One is a 2:26 marathoner, the other 2:31.  The "faster" of the two runs a 16:20 5K, and the other was once under 15:00, had been to Africa to train, etc......very knowledgeable guys regarding running.  One of them said he had never ran 70 miles in a week in his life, while the other said he had been over 70 miles numerous times training for races, but for short periods, but considered those miles to absolutely important.  And then he pointed to the other and said, but I can't swim like he can......and we got into a smaller discussion about cross training.  They both were of the opinion that cross training absolutely CAN take the place of some running for aerobic endurance work, but both also agreed that the cross training had to be able to be done at a very high level.  In other words, if you can swim 40,000 yards per week, and are capable of doing most of your 100"s in the 1:00-105 range then it certainly crosses over and helps you be able to keep the miles off your legs.  As for biking, both also agreed there is benefit there to running, but only if you are riding 50-75 miles and holding high watts and cadence.  Both also agreed that the slow swimming that most of us do will not transfer to our running, nor will the riding that a lot of us do.

Neither said that running less is an answer to faster running, but both said that running more doesn't have to be the answer..... but it's the easiest answer. They said speed/focused work is essential, as is drilling, but neither would commit to what was best for the average runner, saying that speed work is a very individual thing, but both recommended drills.  And then one of them said, "run lot's of 400's, and the other laughed and agreed."

The best parts of the discussion centered around their experience with Africa and African runners.....where "sub 14 minute 5K's are so common that there are people who do it and nobody cares."  That's crazy.

Both of them believed that in order to make serious gains in running it was very beneficial to have a coach who understood that there is no one way that fits everyone and tailors a plan to a person's specific strengths and weaknesses.  And they also agreed that if you put 10 people in a room who had a run background you'd eventually have 10 people disagreeing about something with regard to training and running. 

Pretty interesting night here.

In our discussion, both said that you could, but that you would need to be at a high level of swimming and cycling to have it transfer to something usable in running.....it was a pretty interesting discussion.  Both said that the type of swimming the majority of us do would not transfer to anything useful in running......but that very well trained swimmers got a lot of benefit from their workouts.....both in aerobic capacity and strength of hip flexors from kicking. Still, we were talking about being able to make up those 10-15 miles between 60 and 75 per month with cross training.  Both had ran at least 60 mpw to hit their marathon times (with a good deal of speed work, but both are highly skilled and trained)....but both emphasized that those long weeks were for short duration.  From discussions I have read here that gets lost pretty easily as people throw around the high mileage.....those are usually peak miles.

I definitely see the big aerobic engine helping a lot for shorter distance running where VO2max is often a limiter and the muscular endurance in your legs is not. 

However, in the marathon, you never even run faster than high aerobic pace if you're doing it right (nowhere near threshold or 1-hr max pace), and you barely even get out of breath for 25 of the 26 miles. So a huge aerobic engine isn't crucial, whereas the muscular endurance in the legs is absolutely the critical component. Nobody ever quits a marathon because they were huffing and puffing too hard - everyone quits because their legs cramp and then they're forced to call it a day.

I seriously doubt that the eccentric contracts of running could be compensated for even with vigorous kicking in swimming. You'd get far more run benefits by just adding 5-10 easy run miles per week than by doing 30,000 kick yards at high intensity in the pool.

I agree with what those guys said - for ITU racing and even standalone run events up to the half marathon. But I really think the marathon is a different beast. Honestly, unless you've actually done it yourself, you really have no idea how different the challenges of racing a full marathon are compared to triathlon racing.  Lots of triathletes can 'fake' a fast half marathon by x-training, but none of them can fake a full marathon (run as fast as a legitimate Mcmillan estimate).