This is cumulative across all your workouts.
The fastest way I've found to "burn" calories in the game is to create a custom workout with the Cardio Boxing activity. Then, when the activity starts, instead of aiming for the punch targets, start bounding side to side from foot to foot, while also flailing your arms out and in (as if alternating left and right hooks). Doing this should net you about one calorie per second.
You'll hit a few targets as a matter of course and others will time out, but you can do this to spread out your workout over several minutes and log several hundred calories per workout. It can still get pretty tiring but for effort spent to time spent it's probably as good as you'll find. Of course, I'd love to hear any other tips you might have.
(One slightly annoying thing is that since you won't be doing well at hitting targets, the tutorial might pop up once or twice. Have a controller handy so that you can dismiss this as quickly as possible).