There are some inaccurate assumptions here. One of the biggest assumptions made was that what's "life-affirming" (or "worth it") to you is life-affirming to other people. Perhaps the half of the people who didn't immediately respond with "I'm in" simply don't value the 5K run as much as you? Perhaps they already thought about childcare difficulties and decided that they'd much rather spend their time with their family than training for a 5K run. Perhaps the 5K run was optional to them, and child rearing was not.

You assumed that getting the details before a commitment always scares everyone away, but large projects requiring lots of resources don't scare people who know how to break them down into smaller tasks and milestones. Furthermore, knowing what has to be done (for a commitment) beforehand allows the tasks ahead to be better managed, which removes a lot of uncertainty.

I interpret the Kenneth Blanchard quote differently. To me, he's just saying that if I'm committing to something, then I should make time to make it happen. The quote doesn't say anything about getting the details first or committing first. When I really commit myself to something, I can find time to make it happen, and knowing the details of the commitment beforehand doesn't preclude me from doing so.