+1

Calories burned/ time can be very closely approximated as a function of heart rate over time. If you do a brisk walk at 120 BPM, you will burn more cal/min than a leisurely walk at 90 bpm. Here's the kicker. Since you are traveling faster at 120 bpm, you will cover more distance than you will at 90 bpm. This means that Cal / distance is roughly the same.

A bike is different in that if you are doing les than 12 mph, this calculation goes out the window because the amount of effort drops drastically if you are not constantly pedaling. Over 15 mph you will be pedaling constantly and the Cal /distance will once again become about the same.

At a given heart rate, say 120, you will burn the same amount of Calories/time on a bike as you will walking, but you will go much farther on a bike. Conversely you will burn more calories per mile walking than on a bike, but it will take you longer.