Listen, I am a pretty fit but too fat cyclist weighing close to 200 lbs. On a flat course with no wind I can ride 24 miles in an hour. At that pace I am absolutely exhausted at the end onpf the hour and have burned almost exactly 1000 calories.
Burning 2000 calories in three hours is very hard. The algorithms used by most HR monitors are hopeless, and hopelessly overstated. For most purposes, you may as well estimate 25 calories per mile. You won't be far out and it'll save you a lot of expense on a powermeter.