How do I make my walking animation to continuously move instead of stopping at the end of the frame As you can see, for some reason when my walking animation ends, why does it take longer for the character to move its legs apart again (that is from frame 0 to 40) compared to when the character moves apart its legs again from frame 80 to frame 120?

How do I continually get the character to move its legs repeatedly?
You can do that by changing how the interpolation is in the Graph editor.

1.Open up the graph editor and make sure you're on F-Curves;

2.Select all necessary keyframes and press Shift + E;

3.Now a little menu will appear, click on "Make Cyclic (F Modifier).

There, now your animation should be what you want.