LEGO Spot Micro
Spot Micro was developed using our interactive 4-bar walking linkage optimizer, which you can use to modify the linkage here, and below is a video of the simulator in action.
Spot Micro's linkage has a fairly consistent foot-speed, which makes it a good candidate for rotating the crank by driving the legs, as is done in the following treadmill video:
Version 2 below is slightly bigger to eventually accommodate motors and RC controllers:
The following video includes step-by-step instructions for building the smaller version 1: