When I read the title the first thing that came to my mind were Magna Tiles. Glad they made it on the list.
It's the only toy in the house that lasted the test of time from she 4-8 (and counting). Also I love tidying up Magna tiles, even that is fun!
My oldest kid got a small sample of Clixco and was surprisingly entertained even with a limited set of possibilities they offered. They're great fidget toys as well.
Nice idea but I think the predictions are way off. I ran a 3:17 in spring and targeting a 3:10 marathon in a couple of weeks.
As unlikely as it sounds Strava predicts 3:13 (think they base it on my last marathon), Garmin is similar. Runalyze is about as off as you are.
Maybe you're putting too much emphasis on weekly volume. M35 and I can run this with 50k weekly and 75k peak volume. Relatively confident I'd be able to sub 3 with weekly volume of 80k/100k peak.
As theoretical advice I would suggest to read the book 'The cold start problem'. It explains how two sided marketplaces work and how you can get them started.
As practical advice I would suggest finding people in proximity to you and asking them if they would like to try it.
The same can be done online, try searching for Reddit threads and other pieces where people who are your target audience might hang out.
Explanation of the pieces would be nice too!