What is your current frame rate? It looks rather low. Standard FPS when animating action is usually around 24 to 30 frames per second, and is a good area to aim at for all animation until you get stylized enough to begin experimenting with lower and higher frame rates.
one thing I'll note heavily is the amount of motion per frame transition. How much has the animation changed per frame? Like how we use high levels of contrast and size to depict the importance of a single space in a painting, animation uses time as a way of emphasizing force and effort, thus also importance. The more spent in a single place doing a single action, the more emphasis it has. If you want to see an example, look at the difference in how powerful a single punch thrown feels between these two studies someone else did in their art blog. The difference was the build up before the punch itself.
Another really good example is, if you've ever played it, the Falcon Punch animation from any of the Super Smash Brothers games. If you were to actually count the number of frames in the wind up and compared it to the number of frames in the punch itself, you'd be shocked. If you then look at how much Captain Falcon's model moves during that entire period, you'll notice he does rather slow movements to pull back his fist, and then the punch itself is a very quick few frames in which his model moves much farther distances, comparatively.
this contrast in speed (distance of change between frames) and duration (# of frames to the wind up vs # of frames in the punch) allow us as viewers to interpret the force behind the action. Even without all the fancy sound effects, you can practically feel the impact, hear the swoosh or crunch, and jump with excitement Add in a little camera manipulation to move with the action, and it's all the more powerful.
So going back to rhythm, if you want to display a rhythm then you have to put time emphasis on certain key points of the motion. If you stand up and walk in a straight line, you'll notice your speed varies. You don't quite walk at the same speed during the entire duration of the walk, it slows down and speeds up between each step as you put your foot down, push down, and pull your other leg forward. The variation in speed reduces the faster you walk/run, but it's always still there. That variation is the rhythm.
In your animation, the legs and arms move the exact same distance every frame, with no pause, thus they have no rhythm. It's completely normalized. Even the feet are always moving horizontally at approximately the same rate, without any sort of pause when the feet first press down. If you're looking for places to add frames, it would be at the moment of stepping.
If you want an easier time practicing, try instead something very exaggerated such as a stagger. If a person is staggering, they have a VERY clear rhythm of one step, pause, second step, pause, that you can emphasize quite nicely. Then you slowly work from there towards normal walking where the rhythm is a bit less obvious, and then onto running.
Also, try animating the character walking across the screen rather than as a stationary loop. Try animating them going from a standing position into a couple steps, then a slow walk, a walk, a brisk walk, a light jog, a jog, a light run, a run, a full sprint, then trying to slow down into a stop. Afterwards, if you want to make the whole thing easier to upload, move the drawings of each frame to overlap each other like they would in a running cycle. The distances their limbs travel and where a character's foot stops should be much easier to create initially though when actually animating the character moving across the screen.
Good luck, and feel free to ask me any questions you might have.
