You can stop a workout by telling Siri "end workout".

I'd expect that you can also start one with Siri but have never actually tried, mainly because the idea of using Siri to start/stop workouts never occurred to me until reading this thread, which I happened to be doing this morning while doing a workout on my treadmill.

How accurate is that speech detection when you’re trying to hold in being sick after running a spicy interval?