Bike to run. Bike helps strengthen your knees, and it's very easy to target Zone 2, which achieves the highest calorie burn rate. This is how I got back into running after a meniscus injury, without surgery.

Zone 2 doesn't have the highest burn rate. It's just easier to maintain it longer. 1 hour Zone 2 burns more than 30 in Zone 3, but 30 in Zone 3 burns more than 30 in Zone 2.