[billroberts]

That's true, but you have to work pretty hard on a bike to get your pulse rate as high as you do by running. (I've done a fair bit of cycling and running over the years). For an inexperienced cyclist, tired leg muscles tend to limit the effort before their heart and lungs do. And it's always tempting to take a rest on the downhills. So I reckon in most cases, you burn significantly fewer calories per hour cycling than you do running. On the other hand most people are capable of cycling for say two hours (if they have time) whereas it takes a lot of training to be able to run for two hours.