Hi there, I've run for an hour which would be about 5-600 cals - I think that's the most I'd do, though, and I do it in the evening after I've eaten. I have done 35 mins on a cross-trainer in the morning, although depressingly despite being knackering that was only 250 cals so I admire your gym ability!- I tend to save all my cals til the evening. So the short answer: yes, I think it's possible.
I usually do, ideally exercise first thing and fast until dinner, but on a work day I have to exercise in the evening and eat really late which is not much fun. I do what I feel like, whether it's a 10k run or 40 mins on the elliptical trainer.