Collect First Worrying about selling later
"There's no word on how Google will market Google Fit"
Google doesn't need the cash to worry about having to market the resulting datasets. That isn't how Google works.
Google collects the data, sorts it into a warehouse, releases a bunch of APIs and lets other people figure out where the value is, before making sure those offering value chuck any ad (or heck, if they really want, direct) revenue Google's way.
Own all the data. Own all the users. In the end, everyone has to pay you for something.
And with the EU courts currently considering whether obesity is a disability, you shouldn't start worrying that the use of such gadgets (and the collection of data) will be made mandatory ... You shouldn't worry at all ...
.. Oh, and happy nearly-weekend, fellow 'Tards ;-)