Try squats. They are simple, they roughly correspond in energy expenditure to not-so-fast running, they do not put too much blow on joints. Squats were used by

Try squats. They are simple, they roughly correspond in energy expenditure to not-so-fast running, they do not put too much blow on joints. Squats were used by Soviet marathon runners for training in winter, when it is too cold outside to run. They squatted for hours. One my squat is approximately 2.5 second. 5 minutes amounts to 120 squats. 30 minutes - 720. I think you'll appreciate the power of 720 squats. ;) Squats are pretty variable. You can put your hands behind your back, fold them on the chest, actively wave them helping squats - all those variation will give you different feeling.