How many calories per hour you burn can vary dramatically from person to person. The only way to get a better estimate is to get a VO2Max test done.

For example, I burn 500cal/hour at my easiest pace. At a hard pace I burn 1000 calories per hour and I weigh 120lbs and am a pretty petite woman. Somone else my size might burn half that much - it has to do with effort and natural metabolism.

You can imagine that my main race strategies center around food. But I know other women I race against who are good with about 200 calories for a 2.5 hour race ... a race where I consume 1250 or more calories (you can only absorb so many calories per hour before your digestive system says 'no way' and shuts down on you).

Going by 'feel' to ensure that you don't bonk and that you recover well is the next best thing to geting a VO2 Max test where they measure blood sugar, etc done.