school me on back-to-back long training runs

I'm still a relative novice ultra runner, but running on tired legs DOES teach you exactly that.  As you fatigue, your gait changes and you start firing different muscles.  How do you build up those muscles to carry you for 10+ miles if you're not stressing them in training?  You don't get 2-3 days of "recovery" between the first 20 miles and the last 20 miles on a 50-miler, so how are you going to get through those last 20?  Back-to-back long days let you push longer before the fatigue sets in and really slows you down.  Physiological adaptation?  It really is there, and you're far less likely to be injured running 19+9 (28 total) than trying to run 28 straight. 

As you build towards 50 or 100 miles (or beyond), it gets very difficult to "train" at close-to-race-distances, especially to try doing the long run as a single run.  My friend training for the 100-mile run would do 25+ on Saturday and then another 12-20 on Sunday. 

I agree with the long back-to-back long runs for all the reasons stated above. Once you've done an ultra or two and put in the proper training miles, your whole mindset as to what is considered "long" and what constitutes "recovery" changes.

As for chance of injury, I think it's minimal. A serious ultra runner already has developed the cardio base and physical adaptations to be able to handle the long back-to-backs. It's like anything else; you have to build up to it.

You're not just training to run 26.2, you're training to run 4 times that far, so. After awhile you'll scoff at *just* 26.2.

I'm not an ultra runner but I've been training for my first 50 (less than 3 weeks away).  The one huge thing I've noticed is that running on tired legs has helped me run better on tired legs. It's also helped tremendously mentally.   Before, if I had been tired or my quads were beat I would just stop.  Now I just keep going.  I haven't been able to do those really long back to backs like you mentioned in your schedule.  I did decide that IF I EVER decide to do a 100 miler (or another 50 miler for that matter) I would throw them in here and there.  Perhaps not every weekend, though.

I did read somewhere that doing those back to back long runs helps with running on an empty tank.  Not sure about the science behind it, though. 

I guess I'm just winging this 50 miler ;-)

As far as injury goes,  I think if you have a good base and your smart about your increase in miles and careful with your speed work injuries can be avoided.  I had one setback in training and I can tell you exactly why it happened - I was being stupid!!!

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I have been doing ultras for the last several years and I have never done back to back long runs, so I do not necessarily think that they are required.  Of course they will help you in the later parts of an ultra.  Be careful becuase if you do not have the base to support the long runs, then you do risk the chance of injury.  I have seen several ultra runners get injured becuase of over training. 

I see a lot of assumptions in the answers (and in my opinion in the OP)...

The "of course it helps because it simulates what happens in the race" type mentality used to be very prevalent in marathon training... if you didn't get out once a week and run 20-23 mile runs during your peak, you just weren't really serious about training for your marathon.  Anecdotally, I've seen far better results (in me and in friends) from people who backed off on the long run but ran more miles throughout the week.  I just can't help but wonder if the physiological gains to be made by running two 20 mile runs with 3 days of easy running between them wouldn't make me a better runner on race day than the back-to-back runs.

Does anyone know if this has been studied - like two groups of runners both running the same weekly mileage, but one with and one without back-to-back long runs?

This ^^^^ seems spot on in my opinion.

You'll ultimately go with whatever you wish to justify, but the above has worked for me and I have two friends that have done 100 milers and they both adhered to the above advice and pass the same advice on. They skipped any "speedwork" but would make sure to do back-to-back long runs (20-30 milers on SAT and 10-15 milers on Sunday) but they would work their run/walk schedule into the training runs just like on the 100 miler day.

Good roads to you!

FWIW, I've never done back to back long runs, and I've completed two 100s, one 100k, and a few 50s.  I'm just not built for that kind of training.  I know my body well enough to know that back to back long runs would more likely leave me injured than anything else.

What works better for me is building up my one long run of the week up to the ~6 hour range, and then taking at least one, or maybe even two, days off for recovery after that.

Also, what better training than racing?  If your goal is a 100 miler, use a 50 miler maybe 4 to 6 weeks out.  I'll take one 9 to 11 hour effort over 9 hours spread across two days, anytime.  Or, do something else that keeps you on your feet and moving for one big long push.  One of my key sessions ramping up for WS this year was a Grand Canyon Rim to Rim to Rim.  Toughest ~47 miles I've ever done.

For me, it feels like I do better if I "do all the damage" in one big push, and then let my body recover.  I prefer that approach over a big day, and then another big day, when what my body really needs today is to heal from yesterday's big day.

We're each an experiment of one.  You know yourself better than anyone else, both physically and mentally.  Do what feels right to you.  There's more than one way to get to the finish line of an ultra.


 

Hey I just finished my first 100 - 2 weeks ago and felt good the whole way. This is my 2 cents on back-to-backs

this is all based on time
2 day total run mileage should be arund 45% of weekly mileage
day 1 of b2b should be around 55-60% of 2 day volume then making day 2 around 40-45% od weekend volume.

What I learned from running a 100- 2 weeks ago when I finished 15th out of like 280 in 21 hours I still walked around 40 out of the 100 miles. I paced 3 min run and 3 min walk from the start through 50 miles. After the half mark I went to 6 min run 3 min walk through 91 miles then ran as mush of the last 9 into the finish.

One thing I would do differently in Back to back days is more walking on day 2. I would do my normal long run on day 1, which includes walking but then on day 2 I might do like a 3 hour run in the am then go out for a 3 hour walk in the PM. or include that in the am. I never realized how much walking you do in a 100 till I actually did it.