You start with gross efficiency, which for humans is going to be 20-30%. So when the tool says you expended 1200 calories and you assume an average efficiency of 25%, that means you produced 1200/4 = 300 kcal of actual work and 1200-300 = 900 kcal in excess heat.

The 300 kcal of work equate to ~349 Watt hours as WolframAlpha will tell you, so if you want to produce that in a single hour you need to continuously put out 349 Watts.

Now we have put lots of humans on scientific ergos which can accurately measure work, and that is a very high number for a single hour for any individual. There is a chart here that gives you the kind of work rates achievable by category:

http://d4nuk0dd6nrma.cloudfront.net/wp-content/uploads/2009/...

It's normalized by weight but it's more accurately height, someone weighing 300lbs isn't suddenly going to produce more power than at a normal weight. The number you're looking for is "FT" or Functional Threshold, which is generally assumed to be around the absolute maximum average power sustainable for an hour. At the end of that you should fall over and puke your guts out. So for someone doing 350 Watts for a hour, that would put them at world class or domestic pro level.

They would have to chug harder than a train to burn 3,000 calories on an elliptical. Chances are it's less than 1,000 calories [1].

[1] http://well.blogs.nytimes.com/2015/08/07/walking-vs-elliptic...