More than two hours. Looking at the Runner's World calculator for a comparison:

Inputs are: 1 mile, 11:30 minutes/mile, 210 lbs weight

Gives back 159 calories for a 1 mile run, 829 calories per hour.

My Beat Saber workouts yesterday tally up at 830 calories per hour and the amount of sweating felt in the ballpark of running an 11 minute mile. I don't think the watch's estimates are that outlandish.

https://www.runnersworld.com/training/a20801301/calories-bur...

Again factoring in that I weigh 210 lbs, their calculator also says a marathon would be upwards of 4100 calories for me. 1900 calories clocks in at less than half of a marathon.

It’s certainly possible to burn 800+ calories per hour for a couple hours. Normal, actually, for someone doing athletic training. (E.g., anyone training semi-seriously for any kind road race is doing this at least once a week.)

But I’d pretty surprised if you did it without training to build up to it. You could have developed the aerobic capacity through other activities, but I’d have thought various muscles would be unprepared and would be giving you rather strong pain signals. If you nevertheless pushed through that, I’d think you’d be SORE AF today and barely able to move.

Or maybe your activity confused the calorie model your Apple Watch uses.

Not that it isn’t great exercise.

I exercise bike pretty much daily (typically 30-60 minutes), so my arms are flimsy but in terms of cardio capacity I'm not in awful shape. My arms are definitely sore today.

That's apples to oranges. A regular runner who runs a few times a week could do 11:30 minute miles just fine. Someone who just started running would have much difficulty doing an 11:30 mile period let alone sustaining that pace for two hours.

My Garmin Fenix 3 (with HRM strap) reckons I used 2600 and 2900 calories for my 2015 marathons at about 190lbs. 4100 seems excessively high.

Found another reference that estimates 3600 at 210 lbs, so maybe somewhere in the middle:

>According to the Cleveland Clinic Center for Consumer Health, a runner who weighs 130 lb. will burn 2,224 calories during a marathon, a 165 lb. runner will burn 2,822 calories and a 210 lb. runner will burn 3,593 calories.

https://www.livestrong.com/article/302836-how-many-calories-...

Regardless, my main takeaway is that I’m going to burn more calories with this than I was without it.