Not all cardio is built the same and walking/running (for many) is not the most efficient way to burn calories. Most "calories burned" are also an estimate, of

Not all cardio is built the same and walking/running (for many) is not the most efficient way to burn calories. Most "calories burned" are also an estimate, often based on heart-rate and other body parameters. I burn anywhere from 500-800+ calories per hour when cycling (depending on intensity, based on actual recorded power). I'm fairly light (63-64kgs) so my power (and as a result energy expenditure) will be less than that of someone larger. An endurance run for me will burn much closer to an estimated 300kCal/hr. While I'm no longer a runner, even at peak I would run only 4-5hrs/week (and usually no more than 1:30-1:45hrs/run). I readily cycle 4-5 hours though and burn 3000kCal+. So if goal is to burn as many calories as possible, I would consider cycling and swimming, since both burn a considerable amount and can be done for longer stretches of time without major body impact. The unfortunate drawback of cycling is the lack of impact that makes it easy to cycle for long amounts. Have to balance that with other forms of exercise to stay healthy.