[sasawpg]

At 25% efficiency (on the high side), 1000kCal is about 1050kJ, which is just shy of 300W. Most adults can't sustain 300W for an hour. In addition, reasonable riding conditions (flat, low to moderate winds) will get you moving faster than 20mph at 300W. Most adults are definitely not burning 1000kcal riding a bike for an hour and even fewer are doing that rate of calorie burn for longer than an hour. I regularly have >1000kcal rides, but I have yet to hit 1000kcal for a single hour at my highest intensity. I can do close to 300W for 20 minutes after which time the power drops off rapidly. Larger cyclists that are trained can absolutely sustain 300W for an hour, but won't do this without a significant training effort.

I don't mean to imply that you won't burn a lot of calories cycling, quite the opposite, I think it is one of the easiest and gentlest ways to do so. You are however overstating what most non-elite cyclists can regularly accomplish in terms of calorie burn per hour.

Source: I have averaged 450hrs/yr for last 3 years and closely monitor my data.