This morning I was checking my data in the health app, when I noticed two new sections: Symptoms and Mobility.
Both could be useful, but as a walking enthusiast, the Mobility metrics caught my eye:

Four of the new mobility types, Step Length, Walking Asymmetry, Walking Speed, and Double Support Time have a paragraph similar to the screenshot above.
That means that no external device is needed to collect this data, as long a I carry my iPhone in my pocket, which I always do.
One question remains though: How can I activate this function? I found no options in the settings. I guess I’ll have to wait for the final release to find out…