Jason Snell writing at Tom’s Guide:
The Workout app remembers your most recent workouts and offers them as Quick Start options at the top of the list of workouts. So if you run or walk specific distances regularly, you’ll be able to start them with a single tap. While you’re working out, if you want to mark a segment (so you can split your workout up into chunks, each with its own distance and pace calculations), tap twice on the Apple Watch display during the workout. If you need to pause a workout temporarily, just press the Digital Crown and the side button simultaneously. Push both buttons again to resume the workout later.
I’ve been using the Workout app on watchOS 3.0 since it’s officially come out of beta and I had no idea I could mark a segment or pause the workout by pressing the Digital Crown and side button simultaneously (and press both again to resume). Thanks for the tip Jason!