Place the WALK MOVIECLIP on one side of the screen. (Preferably on the opposite side he is facing which depends on which way you made him face)
For the sake of argument, insert a key frame on frame 20.
Now "move" the WALK MOVIECLIP to the opposite side of the screen.
Right click the timeline anywhere in between frame 1 and 20 and click CREATE MOTION TWEEN
This creates the "In betweens".
Meaning, it will complete the movements of the WALK MOVIECLIP "in between" frames 1-20.

You can move the playhead back and forth to see it moves from one side to the other. However you wouldnt be ablre to see the man "walking". as that is on the other timeline. But we know that works.
All you have to do is test the movie again.

Phew!