How come walking converts chemical energy into mechanical energy

Walking requires mechanical energy, because it involved accelerating and lifting a mass. That energy has to come from somewhere. Your body isn't built to run on solar panels, or a windmill, or a nuclear reactor, so the only place you can get the energy is from putting food into yourself and letting your stomach and guts do chemical reactions with it.

Chemical energy (calories) is converted by your body walking on the surface into mechanical/kinetic energy