Track-walking nanomotors and larger systems integrating these motors are important for wide real-world applications of nanotechnology. However, inventing these nanomotors remains difficult, a sharp contrast to the wide-spread success of simpler switch-like nanodevices - even though the latter already encompasses basic elements of the former such as engine-like bi-state contraction/extension or leg-like controllable binding. This conspicuous gap reflects an impeding bottleneck for the nanomotor development, namely lack of a modularized construction by which spatially and functionally separable 'engines' and 'legs' are flexibly assembled into a self-directed motor. Indeed, all track-walking nanomotors reported to date combine the engine and leg functions in the same molecular part, which largely underpins the device-motor gap. Here we propose a general design principle allowing the modularized nanomotor construction from disentangled engine-like and leg-like motifs, and provide an experimental proof of concept by implementing a bipedal DNA nanomotor up to a best working regime of this versatile design principle. The motor uses a light-powered contraction-extension switch to drive a coordinated hand-over-hand directional walking on a DNA track. Systematic fluorescence experiments confirm the motor's directional motion, and suggest that the motor possesses two directional biases, one for rear leg dissociation and one for forward leg binding. This study opens a viable route to develop track-walking nanomotors from numerous molecular switches and binding motifs available from nanodevice research and biology.