Cycling is actually very good. These are my average calculations: Cycling: 25kcal for a kilometer. A nice riding speed is like 20km/h with stops and all. So for a 50km ride that is two and a half hours, and around of 1000kcal spent. Also very low impact. Running: about 60kcal a kilometer, and a nice jog pace is around 7km/h. Two hours and a half running at this pace are 18km, with around of 1100kcal spent. It is a bit higher impact as well. So both are similar measured from the calorie consumption.  Goes without saying, a fatass cannot do either one. Basically walking at 5km/h for an hour is exhausting for such a lad. I compare it to carrying a backpack with 100kg. I would be  tired after 5km. I do have respect for my knees so I wouldnt do that to myself...