Two 10s?

Okay, I'm going to squeeze in the 20. I haven't missed a single workout and I don't want to start now !

Toddler just went down for a nap so at least he'll be sleeping during a lot of my run

Good job! 

The thing to know is that on long runs as you're building up is that training benefit is all in the last few miles.  You can't get the training stress of a 20 mile run and the corresponding adaptation if you short the workout to say 18, or if you switch it to two 10s (although that's better than shorting it).

it's also common to try to switch a longer easier run for something harder at a shorter distance.  But if you had a 20 scheduled and you want to say do a harder 16 mile run, it's not going to be the same sort of stress. And you might injure yourself or some such at a harder pace.

So yeah, it's best to just stick with the long run as planned if you can.

This would have been my view.  IMO, running is running.  I would be interested to know why others feel that two 10's is less than one 20?  What type of additional training stress do you get? 

And if running more in one day does create more training stress than breaking it up into 2 days, how much more do you have to run over 2 days to make it equal?  IOW, does 10 and 12 on consecutive days equal to one 20?  10 and 14?  10 and 17?

I find that two 10 mile runs in a row is far easier than one 20 mile run due to intermediate recovery.  I find that a 200 mile bike ride is a lot harder than back to back centuries.  I think this is common sense.  You run 10 miles, you get home, you take a shower, you have a beer, you relax, you get a good night's sleep, you have breakfast, and then you go run 10 more miles?  How can that be the same as running 20 miles in the first place?

There is a school of thought that says "it's total volume that matters, not so much the long run".  There's another school that says "volume and distance both matter", and I'm firmly in the latter group.  I think nothing substitutes for distance.  When it comes to running long distances, I say that 2x10 doesn't equal 20, and neither does 15+5, 12+8.   You want to train for a 26 mile run, you have to have some good long runs. 

I feel the same way about the 112 and especially the 2.6 distances.

I agree.  Everyone is different (situations, physical body, goals, etc), we don't have all the facts, and thus anyone's context-free advice is as likely to be useful as the next person's. Hence my auto-signature. 

Let me share one of the phrases I've used for years with my now college-age children that is guaranteed to induce eye rolling: "there are many paths".  

There are many paths in life to whatever success you want to achieve.  Some paths are more likely than others to achieve the goals you want to achieve, some are less likely.  Some paths are well traveled, some paths are lightly traveled.  Some paths are well known yet still common paths to failure.  Some paths are well known but still difficult to achieve.  Some paths require some innate must-be-born-with-it trait in order to achieve success.  Some paths are unique and it is unlikely anyone else will ever be able to repeat the same path.

This is one of those times I'd say there are many paths.  Feel free to roll your eyes.  I'm used to it.

You're thinking like a runner and not a triathlete.  We don't train for a 26 mile run.  We train for a 140.6 mile accumulated swim, bike and run.  There is a difference.  Marathon training, for example, requires so much long running not so much due to "getting in the distance" but because it's basically the sole form of fitness.  The fitness only comes from running.  IM training obviously includes swimming and a whole lot of cycling in addition to running.

I capped my long runs at 10 miles this year.  PR'd both the HIM and IM runs this year.  I'm a firm believer in volume through consistent running.

Not really. I'm quite a believer in the concept that triathlon is one sport, not three, so we train like triathletes not swimmers, cyclists, and runners.  Still my training beliefs look more like a combination of those three than something wildly different.  For IM training, my total running volume doesn't come close to my open marathon total running volume.  I do believe in the 20 mile run during IM training; not as many as during open marathon training, and no mid-week 14 mile runs and that sort of thing.

And I too believe in volume through consistent running.  I don't think one or two runs per week including a long run is sufficient for IM any more than I think 5 runs per week all of them 10 miles or less is sufficient.

In the past, I tried a couple IM with my long training runs at only 16 miles and I had worse IM runs than when I go the distance.  For me, nothing substitutes for distance.

Exactly my line of thought.  I completely understand that running 2x10 over 2 days seems easier than one 20 mile run.  But is the 20 mile run harder because you're making greater fitness gains?  Or is it simply harder because you're running in a state of greater glycogen depletion, probably a little more dehydrated, and possibly pushing yourself close to or beyond your fitness capabilities?

The OP posted in the context of what she should do to maximize her marathon training given she has prior marathon experience and also has other long runs planned.  So when posing the question of whether to do 1x20 or 2x10, I felt the it was asking which (if any) would produce higher fitness gains.  My answer was...it probably doesn't matter.

I know that a double century is harder than 2x100.  But that doesn't mean it provides more fitness gains.  Lets take the example to a little more of an extreme.  If I wanted to ride 400 miles in one week, with the goal of increasing my cycling fitness...would it be better to ride all 400 miles at once...maybe in a 25-35 hour non stop ride.  Or break the 400 miles up over 7 days...maybe about 70-80 miles each day?  I know the 400 miles in one shot would be harder (glycogen depletion, dehydration, sleep deprivation, etc.), but would it lead to greater fitness gains?  Probably not. 

This is why I asked at what point is two runs equal to one 20.  If 15 and 5 is still less than 20...what happens physiologically in the last 5 miles of a 20 miler that gets you additional fitness gains?  If during a 20 miler your HR increases from 130 the first hour, to 140 the second hour, to 150 the third hour, you can still simulate the time spent in those HR ranges with equal amounts of time in each zone with a 15 and 5 mile run (or 2x10, 12 and 8, etc.) by tweaking the intensity.

There are definitely more than one way to reach a goal.  In no way am I saying that you shouldn't run long.  There are benefits of learning mentally what type of paces you can hold, how you deal with nutrition, and how you adjust to temperatures possibly getting warmer the longer you run.  But I think a lot of people get caught up in "the long run is more important than any other run" train of thought. 

I say this in very general terms.  Someone can dig up some study if they want to that proves the long run provides 3% more benefit than running an equal distance over 2 days if they want to.  I'm trying to dispel the notion that some people tend to believe the long run provides something like 30% more benefit than running equal distance over more days (Bruce, not saying this is what you think, but there are others who seem to think so).  Hence why you see hundreds of threads asking about how they should structure their 16, 18, and 20 mile long runs because they don't want to give them up...but they only run 15-20 miles during the mid week.  Not implying that's the OP (I know the OP puts in her miles), but it's true that 90% of the "what should I do about my running" threads always emphasize the long run...and all the other miles tend to get overlooked as if they don't matter.

Just my 2 cents.

Tell me about it!

DP