Below is the basic flow of the body for a push. This is a more exaggerated, cartoon feel over a realistic but the premise is the same.
As for the feet. You can see in pose 3 that one leg is bent and the other is practically straight. With mine, I checked some video reference on the push and found that people tend to twist the foot on the straight leg slightly for friction or balance purposes, therefore I added this to mine. Much like the hands, I wanted the start and end to be the same so set the frames for them and then created the keyframes for the pose below at the same frame as the hands hitting the box.
Also in 3, you can see the characters back bends to reach the box and due to the weight behind the object requiring more force. Due to the way the rig works, when the back moves the locations of the hands move with it, therefore when I positioned the back and set the frames for them I then needed to reposition the hands again. Lastly, I repositioned the head to be looking down as if he's struggling to push the box.
Since I set the first keyframes as the last keyframes too, the animation resets back to its starting position. When I played this through I noticed the front foot would slide back to the start position instead of staying where it was and letting the rear foot lift up to its start. To negate this I keyframed the main control moving forward to keep the foot in place which remedied the issue.
The final animation can be seen below.