Given weight and road grade plays a role, they don't need to average 50km/h necessarily. Still, 1kcal/hour seems quite intense for a regular exercise for me, but similarly large professional-level endurance athletes probably burn far more during serious training or competitions. 750kcal/hr seems manageable to me as a non-athletic person (supposedly I've burned 2340 calories over 2.7hours once... but I was totally wiped out afterwards IIRC). 1K/hr isn't something I could maintain for more than about 1.5 hours even on the best of days.