Originally Posted by mr_pedro

Heart rate zone does not always give the same calorie expenditure, it depends on activity.
Running 2 hours at a 6 min mile pace requires about 2200 Calories for a 155 lb person.
The 100k in 4.5h, is that in km, amounts to about 14 mph. That will require about 850 Calories in the same 2 hours.

Also, you mentioned using a tandem, these are much more efficient than the rugular bike figures above. There is twice the power with almost half the drag, meaning even less calories required on the tandem.