Any data supporting high frequency/low intensity running?

I agree with what Scout is saying. Your proof is really how the successful racers have trained. Pick up a book like 'Running with the Legends' by Mike Sandrock. Great stories in there of how the greats trained. You will see first hand that many have trained every day for years to make improvement, be successful and to win at the highest levels. 

If you can convince them to even run 10 minutes on days 4, 5, and 6 of the week, it will make a difference. Hope this helps somewhat. It's hard to convince people when they already have their minds made up. 

The results speak for themselves. Virtually every top distance runner at every level uses the same approach. Run lots, mostly easy, sometimes hard. The run lots part of the top runners often exceeds 70, 80, 90, 100+ miles/week for long periods of time. Mostly easy is 60-120 seconds per mile slower than race pace and makes up 80+ percent of the volume. The higher intensity stuff makes up the balance and varies depending on the race distance. Top people in 5K to marathon often are very close in the number of miles run, only differing on how they do their faster/harder efforts.

Running every day is actually much easier than people think. It allows for breaking the weekly volume into smaller chunks and when those chunks are done mostly easy there are no real recovery costs. Plus it's easy to build volume simply by adding just a little very day.

The thing to ask your friends is what Dr. Phil says:, "How's that working for ya?" If they're satisfied with their results then they don't have much motivation to change.

30 years ago we called it the "Lydiard Method".  There are probably tons of studies supporting the value.  But you sound like what you are really trying to find is a study that compares every day easy mileage for a triathlete when compared to the more traditional 3x per week runs.

I don't think a study like that exists.

I am pretty sure there is a study released this year that shows triathletes have more running injuries than runners.  That would be strong evidence that 6-7 days of running is less injurious than 3-4 days. 

Triathletes feel that every run must be a 'quality' run because their training schedule only gives them 3 or so runs per week.  There's LONG, there's SPEED and there's TEMPO.  Boom, hurt.

Runners know that most of their runs must be easy.  And those runners that are doing 2 or more workouts per week that are a little harder have usually worked their way there over years of dialy running.

Not scientific...just "n=1"...but there's no question in my case I've had more injuries running 3 - 4 x week as a triathlete than when I was running 7 days/week previously.  I don't find it to be a function of the "hard" runs (threshold, interval, hills) as much as weekend long runs not supported by enough volume the rest of the week.

I'm unsure as to the literature on this as well, but add me to the list of believers.

I recently switched from running about 4 times per week to running 6x, and I'm feeling amazingly good.  As recently as September I was having piriformis/hamstring issues, shin issues, and a knee that liked to complain.  Since switching over into running nearly every day?  NOTHING HURTS.  I know it's only been a month or so, but I'm hooked.

Lydiard:

http://www.fitnesssports.com/lyd_clinic_guide/Arthur_Lydiard.htm

Good free source to get you started.

I like that, Daniels Running Formula and then Pfitzinger as the 3 main running sources.  They have it pretty well covered.

I'm a believer myself. But unfortunately, the "everybody's doing it" argument and the "top runners do it" argument don't give us what the OP is looking for. The first argument doesn't show much. And the one about top runners doesn't tell us how to translate the training of a 100-120 mpw pro into a plan for a 30-50 mpw AGer. Copy the proportions of high/low intensity work? Or copy the "key workouts", thereby increasing the proportion of high intensity work? Either seems like a reasonable extrapolation.

I can't offer any controlled data to help. I imagine that it would be difficult to study the question that we're interested in here, especially because questions of injury susceptibility tend to require much larger study samples than are needed for questions about VOTmax increase and its ilk. 

The argument that I find most persuasive in regard to injury prevention is the observation that the body fares better when stresses are not concentrated in a small number of bouts each week. Reducing the stress of each individual workout, and following a plan that requires rapid recovery from each workout, is an easy concept to grasp.

Then there's the separate question of whether there's any benefit beyond injury prevention. Folks are naturally skeptical of the idea that you can get faster by just running easy. (And it doesn't help that we triathletes are singing the praises of hard intervals for biking and swimming.) Perhaps a key here is that the body's ability to sustain prolonged pounding is a key limiter in running, in addition to aerobic capacity. That is, presumably, why running for 3-4 hours in a marathon is harder than biking at an identical HR for the same length of time. 

Honestly, I'm not sure a study would help much anyway because of a lot of things Colin mentioned. These guys have the preconceived notion that they need to work hard and that reducing the daily mileage (even though weekly would remain or increase) is seen as a step backwards. They don't want to say that their run each day is less than what they have been doing. After so many years of the "no pain no gain" mantra, it's hard for me to believe just how good I'm supposed to feel after most of the runs (and I know I'm not the only one). And that I should still have something left over after the hard ones.

Unless someone is really open minded they will tend to believe information that tells them what they want to hear anyway.

Something that may work is to continue doing what you're doing. Show that it does work. Let them keep going, see what happens when the plateau. Ask them what they're going to change to keep the improvements coming. Some things just take time to believe. This approach isn't that different from running itself. Consistent improvement over the long term. Not trying to win something immediately.

(I'm not disagreeing with you, just using this statement as a launching point.)

Or are they afraid of what is truly hard?  Running every day is very challenging, both mentally and logistically, especially when it is not the case that each run is a (so-called) 'key' workout (an overrated concept, IMO).

(I don't exempt myself from this problem.  I used to have it in running, and I still have it in the pool.  I talk myself into the idea that erratic and short, though somewhat intense, sessions in the pool are just as good as more consistent training.   It ain't so.)

Injury -- it's a no-brainer.  Really.  Someone already said it:  the same mileage spread out over more sessions is less, not more, likely to lead to injury, especially when the intensity is (overall) less.

On the issue of actual fitness gains it is easier to see how one might be skeptical.   I have one theoretical point to make, and one practical suggestion.

The theoretical point is that running easier most of the time allows one to make the hard sessions truly hard.  If you beat yourself down every time you run, there really is no such thing as a 'hard' session.  (They might feel hard, but that's because you are beat up, not because you are truly going hard.  There's a difference between 'this hurts' and 'I am putting a very strong effort'.)

The practical point is that it really isn't that hard to learn that it actually takes very little speedwork (in whatever form) to reap the benefits of the 'sometimes hard' part of the truism.  (That's only part of the story, I agree, and the other part is more about the long-term and so probably more difficult to learn.)  Honestly, I think that if most people tried it for a month they would see the point.  Go one month running 6-7 times per week (however short is necessary to achieve that frequency).  Twice during the month, do a hard session (intervals, tempo, I don't care) that constitutes around 3% of total mileage for that week.  Yes, that's short, but do it HARD.  Everything else is ridiculously, embarrassingly, easy (as in, "I don't want my grandmother to see me running this slowly" easy).  Honestly compare yourself at the end of that month with yourself at the end of a month (equal mileage) where you ran 'hard' every time to 'make it count'.

I really do think that a month is all it takes to see the difference.

I know it has been said before, but it is really hard to run easy.

2 days ago I did an hour with my wife.  Her easy run is around 13 minute mile.  She was doing 12-13 minute mile for the first 45 minutes or so. (Later she put in some really good fartleks and got below 10 a few times).

I was able to stay with her for the first 1/2 mile, but slowly I would pull ahead.  I stopped for a few minutes a few times, and walked 1/4th of a mile at another so not to get too far ahead/behind. 

I was able to run around a 12 minute mile at the low end, and that is what I wanted... to have someone to pace off of to go slow.

Today, I ran by myself.  I tried to repeat that easy run.  And, I did better than normal.  But I still couldn't go easier than a 10:34.  I started at 12, but the next time I looked at my watch I was sub 11.  I slowed down, but the speed creeps back up.

For comparison my last (really my only) stand alone 5k was in the mid 9s.  My really hard efforts can break below 9, but not for a sustained effort.  But I feel like I get injured running that hard, and right now I'm aware of a number of tender spots (knees and shins monstly) that I think might be from running too hard when the weather cooled down.

So to the OP on top of wanting to race every run, and wanting to brag on the numbers (and I have some guilt in both of these), it isn't easy to run easy... at least not when you are a relatively new runner.

This is a bit informal to call it "data" - we're often reminded that the plural of 'anecdote' is not 'data' 

But the coaches who run the endurance sports group at my Y are strong advocates of high volume, with almost all low intensity. There are a lot of very accomplished triathletes in this group and it has certainly worked for them. Once a week they will do a longer run, still at fairly low intensity. The other runs are almost always intervals that include a lot of aerobic, comfortable chatting pace, with short intervals of higher speeds - often reverting to a short walking recovery after going speedy. The other thing they do is once a week run-focused strength training and muscle activation drills.

One thing this regimen has really helped me with is improving my technique (although that is partly just a function of running with coaching feedback). But I do believe this approach creates the best results with the least amount of wear and tear. Important to keep this last factor in mind, because nobody's very fast sitting on the couch with an injury.

I am quite stunned.  I recently have adapted to a 6x week running schedule from doing years of 3x - 4x a week.  Totally counter-intuitive.  I feel great!

Then explain to them that if they are getting hurt running 3 times a week, then they are running too hard and their bodies are not able to do that type of running.

It's basically all that was done to me. I had the exact same reservations.... and then it was explained to me.  I thought  I was just going to be running the same way I was twice as much. Obviously that is not going to make much sense.

When it was explained, and I did it, it makes perfect sense... I didn't need a study or any scientific data... I was just willing to give it a try. I'm a believer.