Defining “health” isn’t easy, but some definitions do revolve around function, and there are few things more functionally important than walking.
There is a test which shows up in medicine called the 6 Minute Walk test. ( 6MWT). I’ve seen it used in chronic respiratory disease evaluation, rheumatology, obesity, etc, so there is a fair amount of literature on it.
It’s basically a measure of how far one can walk on a flat surface in 6 minutes. Since it is self paced, it’s a relatively safe test, and, most importantly for Citizen Health, it could be a very “mobile friendly, non fungible” test.
Anyway, here’s one article to look at for obesity. https://www.ncbi.nlm.nih.gov/pubmed/23404845 .
I am aware of its limitations - such as for people with certain disabilities - but I think there may be some power to the idea of relating health to a natural function.
Also, I believe there is some evidence that the change in function may be more important than the absolute function for a health improvement measurement - that is, extending you 6MWT by 100 yards could be very important to a feeling of improvement for one’s health in a patient with significant disease.