Cycling/spin can yield more aerobic intensity with less stress on knees, you can't even get to Zone 2 with walking unless you're very overweight. Of course ther

Cycling/spin can yield more aerobic intensity with less stress on knees, you can't even get to Zone 2 with walking unless you're very overweight. Of course there are bodyweight options like aerobics, shadow boxing, jumping rope. Skipping rope would be my favorite were it not for the fact that you need a lot of headspace for the rope. This makes it unviable except outdoors or at a gym/facility.