"I am interested in hearing from 100 run veterans whether this challenge resulted in tangible improvements for them in years past. Did it translate to better race results, or injury, burnout, peaking too early, etc?"
Sully, this will be year six for me personally and I have some reflections to share. My approach in the past is based on the belief that if you want to make a technical alteration in some aspect of your sport (in this case the run) then frequency is the path by which you'll make that change. Frequency - not intensity, not duration.
My stride was way too long and I used the first year to fix that. I set my own parameters on "the streak" (everyone is welcome to do that). My goal was 80 runs in 100 days, 30 min minimum and I allowed (required) myself to walk every 5min for 1 min in month #1. I set the focus point of shorting my stride for every single run and by the end I had permanently changed my stride length (I started with drills, I counted foot falls, I really committed to it). I was 41 years old at the time and ran my first sub 40min 10k that year. By the way, my lower legs were like concrete for the first 3 weeks and I had to do a daily "at-home-physical-therapy" routine to survive it that first bit (stretch, roll, self massage, ice).
Year two was about lifting my heel higher and driving my knee forward. I expanded my goal to 100 runs in 100 days and kept my minimum to 30 min. I was able to maintain the focus without breaking up the run with any walking. I'm a good age group triathlete and this was the fist time ever that I ran over 25 miles a week (funny how you can be a "good" age grouper and never run over 25mi/wk) Side note here: I think was the year that I got behind in runs due to a cold and some international travel and was ~8 runs down with 5 days to go. I was down in South America coaching at the Continental Cups in Vina Del Mar and then La Paz (6 days apart) and I had to run three runs a day while I was there to make the finish.
One year (year 3 or 4) I had the luck of a quick analysis by Bobby McGee (a run coach for whom I have great respect) a few months before we started the 100 day run challenge. He made mention of two flaws in my run: 1) I was too "tall" with my posture to the point where I was nearly leaning my torso back and 2) I wasn't allowing enough rotation in my shoulders. I dedicated that year to fixing those things - upped my commitment to a minimum of 4 miles each run for 100 runs.
Last year I set my focus as leaning forward more from the ankle and, as a result, simply running faster for the first time. I came in with less fitness so I lowered my minimum to 30 min again. I won my age group at the South Beach triathlon just a few weeks after finishing the challenge and that was almost entirely due to the fitness I had coming out of the 100/100.
This year I'm thinking of setting the goal of running more volume. Setting a minimum of 4 miles again but stretching out the mileage in months 2 and 3. I've never run 50 or more miles in a week and I'd like to explore that.
This thing can be whatever you want it to be but no matter what it's a sense of community with great inspiration to get out of one's comfort zone.
Hope you'll jump in, Ian
Ian Murray
http://www.TriathlonTrainingSeries.com I like the pursuit of mastery
Twitter - @TriCoachIan