Last Thursday, former volleyball star Gabby Reece worked out with a bunch of tech journalists on a rooftop in downtown San Francisco. I only made it through half of the 20-minute workout, which says something about my personal fitness level or the workout’s difficulty, perhaps both. The event was to show off some of the features of Fitbit’s new devices. One of those features, a new Cardio Fitness Level function, is actually to determine whether…