Tracking distance over long term in an app

[camelcode]

So I'm creating an app that tracks your progress "walking" from one location to another based on the distance data from the device. In theory, once you install the app, it would start tracking all of the distance that you walk until you walk the predecided length, which could be thousands of miles. 

My problem is that I can't figure out a good way to do the distance tracking. Because the Fitbit doesn't store lifetime data (at least as far as I can tell), I can't just rely on that without having the Fitbit connected to a phone, which seems unideal. 

The other workaround I thought of would be to update the distance data every time the app is opened, but the problem with that is that if the app isn't opened on a certain day, the distance for that day wouldn't be counted. 

Combining these methods would be better, but still not great.

Are there any more elegant, practical solutions?

[Gondwana · Accepted solution]

This may help.