How do I get my iPhone to record an indoor walk?

I use my iPhone to record my workouts on my walking pad. I have an Apple Watch series 5, but the distance and health data are so inaccurate (even when I wear it on my ankle, and even after recalibrating) that I do not use it for that. Instead, I start a stopwatch on my phone and walk my distance, then log that into my iPhone’s Health app as a “Walking” workout. However, when I log this, there is no “Indoor Walk” option like there is on my Apple Watch. It is always recorded as an Outdoor Walk. I do not understand why and I would greatly like to change this but cannot figure out how. Where is the “Indoor Walk” option when NOT using an Apple Watch?